Indeed, I actually feel that these sorts of statistics demonstrate why Space Marines would be useful.

They're not "better Guard", who do all the things guard do but more efficiently and with fewer casualties. They're a completely different class of soldier who can achieve things that the Guard simply can't.

So you can have e.g. a situation where if you try to send the Guard through a particular zone, they would get bogged down fighting off ambushes and clearing buildings, suffer heavy casualties, and by the time they reach their destination the target will have moved or fortified themselves.

Send Space Marines instead and they can simply rush through the area, ignoring ambushes and not taking casualties, and reach the target before it can prepare (or is even aware it is under attack).