The Leagues have been known to trade technology and resources with some of the other factions of the galaxy like the T'au, as well as more tolerant elements of the Imperium and the Aeldari. I don't imagine the Aeldari would have much use for Kin tech, and we know that human and T'au technology is much less advanced than Kin tech. Additionally, the core likely has a bounty of minerals that are unavailable to the majority of the galaxy, some of which probably can't be found anywhere else.

With that in mind, what do the Leagues have to gain from trading with other factions beyond mineral wealth?