Well, drop dilution is the purpose. Even if a universal primary ammo drop gave less ammo for snipers then it did for rifles, it would still be an exponential, massive increase to drop rate compared to what it had before.

and thesedays their Aura Slot is much easier to trade something for than a Mod Slot on their Weapon, so it's something that could be easily considered for over the relatively minor things that their Aura can provide for them anyways. Maybe if they considered Equipping a Scavenger Aura, they'd see that 2.5x the Ammo, is a lot of Ammo. meanwhile Player #258927356235 just complained about Ammo. which are more useful than the Playerbase would ever suggest, but because they don't say Damage on the Mod Card are regarded as impossible of being useful.

It also lets there be supporting mechanics that can be offered, like Mutation Mods and Scavenger Auras.

Like as aforementioned, if there were no Categories to speak of, and just Slot1 and Slot 2, Ammo simply wouldn't exist for quite a few Guns, and if Ammo doesn't matter for almost any Weapons even despite shooting constantly, then Ammo is no longer doing its job of forcing Players to go get more of it. It's easier to control when you have a few Categories setup, whether you set Pickup Scales for individual Weapons or not(which would be nice, sure).