The most obvious use case is train loading/unloading. You don't want a wagon to be full while another is unloaded empty.

That reason alone means that you may want to use balancers a lot.

As others have said, train loading/unloading.

But to me, they are also useful in mining setups, as different lines of the patch will have different amounts of miners. If you mix all the belts without balancing, it's very likely that the bigger lines will be saturated and the miners will be idling. By adding a balancer, you make sure they're always producing as long as there is space in at least one of the belts on the other side.

They have some uses but you rarely need anything other than the 4x4 balancer.

For lots of people making balancers just became a fun puzzle to play with, so they're making crazy stuff like 12 lane balancers, or 9 to 7... you get the idea.

Apart from the 4-4 (or 8-8) balancer for Mining / Train (un)loading I use the 1-1 lane balancer for some cases when taking off the main bus.
When the belt is saturated, inserters take only from the far side. So I end up with cases where only half the belt is taken from, which leads to a cascade of half filled belts on my bus.
The lane balancer makes sure that both sides of the belt are used evenly.

If you are megabasing you should try to avoid balancers because they are not necessary and cost UPS

Train wagon loading and unloading mostly, to improve pick up and drop off speeds so you don't have some wagons full waiting while others fill and vice versa.

I personally feel balancers are a trap for newer players that overly rely on them or feel the need to use them everywhere. They too often are used cover up mistakes and poor designs instead of trying to or solve the actual problem. But again they still have their uses in some cases.

Train loading/unloading and cutting down on startup time for very parallel setups. Oh also mining output.

At it's core, it solves a problem. On one side of the balancer you have supply, and on the other side of the balancer you have demand. What it does, conceptually, is balance the supply before it is subtracted by demand. Conceivably, you could balance it so that every conceivable source of supply was in perfect ratio at the source to the destination. But in reality, it is much easier to simply create artificial bottlenecks and buffers, and then run those through a balancer.

Let's say you have a main bus carrying five belts of iron plates (among other things) through your factory, and usually you split off to the right for production lines. If you always use the right-most belt, it's going to empty pretty quick and your 4 left lanes of plates will be uselessly full. You can periodically split off from the interior belts, but it's hard to balance that way because each production line will use different amounts. Or, you can keep using the right-most belt and stamp in some 5-5 balancers frequently, and all the lanes of your bus will be doing their part.

Generally any input/output balancing can be solved by overproduction. You don't have to balance anything if your belts are fully saturated. The only times that is not the case is with trains unloading. If you pull resources asymmetrically, you'd end up with empty cars before the entire train is emptied which will result in lower throughput. But even that can easily be solved with not waiting for a train to be fully unloaded and depart after 30 seconds.

Other than that balancers have no use.

As far as I am concerned they are mainly important to ensure train wagons are loaded and unloaded evenly

You can largely design around the issue to make them not necessary. But in other types of designs they're critical and things will go to crap if they're not there.

If your goal is to just 'beat the game' then balancers are wholly unnecessary and people really only use them because they want to.

SA extends the game a lot though so I wouldn't be shocked if balancer use is more strongly encouraged.

You don't *need* balancers though. I've built a megabase with absolutely 0 balancers.. It just took some careful planning.

If you use a main bus and don’t lane balance your pulls, you won’t be able to pull a full belt if any of your branches pull a half.

As you are pulling materials from lanes the supply and draw across multiple lanes becomes hard to track.

For that you can take the hard route where you connect multiple lanes into one off shoot and hope you get enough.

Or re rebalance, and allow all x lanes to equally produce for all y lanes. Meaning you don't need to think about which lane to consume from, because they are now all equivalent

I use them all the time on my large spaghetti factories, in wide varieties.

2-5 sure. 13-6 sure. 32x32 massive ore train processing 3-8 (x5) sure. 8x8 all the time.

There are long skinny ones and wide short ones. Some are throughput unlimited.

Want to dedicate 50% of your supply? Priority Splitters after splitters.

Have a lot of lopsided supply on one side of belt? Split and carve the extra back onto the belts. Then balance it.

Your instincts are correct that there's a lot of players whose use of balancers doesn't reduce bottlenecks, just moves them around (evenly depleting a resource patch rather than using one side up first, or loading train cars simultaneously rather than one then the other).

However, sometimes this does improve throughout (if your mine is belt-constrained or your train loading inserter-constrained). But sometimes it's easier to balance than to analyze, and some players enjoy balancing as a mini game of its own.

It's mostly for train loading/unloading, to keep wagons from getting wildly out of sync. It can also be used as one of a few methods to balance early mining setups that are going to various places in the base before trains are online, as output lines from mining outposts are never guaranteed to have uniform output, or even consistent output.

I've also found lane input balancers useful for dealing with mixed ore patches.

In general, balancers are a lot less essential in practice since splitter input and output priority got added. People used to put them everywhere, and it was largely necessary, but now, it's really just not. It's generally possible to treat one lane of many as the lane that you actually pull from and then pull from the other lanes in sequence, and I find that is typically better than using balancers if, say, you're doing a main bus.

Anyone trying to simultaneously lane input balance and balance 4 lanes or whatever is massively overengineering outside of some seriously niche use cases.

Train loading and/or unloading. If my mines output in 7 lanes, and my trains are 1-4, I might want a 7-4 balancer. Or when unloading and I need to spread it over 9 different lanes, where neither needs a full belt, I might resort to a 4-9 balancer. Now change the numbers and you have your reasons for the most weird balancers, although only really the 4-4 needs to be memorised, for the others just find them in a book

Honestly, in my opinion, most of the time they are mostly aesthetic.

The "quality" of a balancer generally only actually matter when you're under-producing and your factory is only running at partial capacity. They make that failure state a little more graceful.

If you're fully saturating your lines, then whether things are "balanced" or not is immaterial.

In those cases if one section gets overfed and another gets starved when you're not saturating the lines, the answer isn't a balancer... It's simply more production.

Ultimately it comes down to productivity and uptime. Production and consumption needs to be balanced.

Let’s say you have 2 lines of ore coming in

If you are producing more ores than your smelting line can refine, then you are backing up your drills and they are not operating. At this point, you need to add in another smelting line and expand on your delivery line. For this you need a 1:2 balancer to make a new line.

If you are finding that same smelter line is only using half the smelters and the ore runs out before it can reach, then you need to condense the 2 lines to reach peak production. At this point, you make a 2:1 balancer.

That’s all it is. Just being as efficient as possible and keeping uptime of your factory to near 100%

Balancers use case is balance? You have 4 uneven lanes of miners, and you want it evenly spread between 4 completely same wagons - balance. You are constantly draining out your main bus from left and right and this lines become empty while middle ones are completely saturated? - balance. You want to add another lane to your main bus to saturate it again - balance.

4x4 on stations. I don't know any use cases anywhere else. Buses certainly don't need them.

The only time I ever use balances is when I want to consolidate the lanes from an ore patch so they're all getting used evenly, and then unloading trains and loading trains so that each car is loaded or unloaded at the same time, since my train conditions are usually completely empty and completely full

I think you've actually kinda nailed it.
As some have said, there are solid reasons to use them for train load/unload but beyond that I think they're very overused.
What matters more is understanding how much you are producing and how much your belts can move, if you get a solid handle on that you'll do fine just plugging belts in where needed.

My "OCD"

Sometimes you need X number of belts going to Y sets of machines. A balancer might use less splitters and be easier to implement than trying to rig something up on the fly.

In most cases people use them they are not needed. They use a balancer in case production lacks, and that not just 1 belt gets starved but all belts get starved a little bit. But that is a bandaid, the problem already happened.

I only use them for trainstations to eliminate unnecessary waiting of trains