[Gatekeeper]

Well, it obviously depends on damage-per-shot, refire rate, reload time, etc. etc. etc. You might as well say "how can a gun with lower RoF but higher damage-per-shot ever be useful unless it's a OSK" - which is an extremely reductive argument.

As for "why use a sniper-rifle for suppression instead of a tank" you could equally well use the same argument for using either for outright kills. The answer is that they have different pros and cons - a tank is a big, obvious target that requires an investment of resources and is limited in where it can go compared to infantry - but in return it's tougher, and has more firepower.

FWIW a tank doesn't need to one-hit-kill infantry in order to be effective either - the PS1 Magrider was a strong anti-tank platform but (relatively) weak against infantry (at least once it's 'magmowing' roadkill potential was nerfed) and was still both fun and effective.

tl;dr - there are plenty of ways of differentiating weapons without resorting to "this one has a OSK while this other one fires faster but doesn't" and OSKs aren't essential to the game.