We actually dont.
Im pretty sure OP meant L1 and R1. These triggers have no function whatsoever while in marksmanship mode, so it would make only sense giving them the function to lean. Even only ONE button would be enough, and its actually all you really need; if you wanna lean the other way, for example, just switch shoulder and then press the lean button, or just press the lean button and then switch shoulder.
It makes so much sense that someone ends up asking themselves how G1 thought it was a good idea to just slap an automated system on that. Its not enough. You are not gonna want to lean only when you are right behind a wall (literally touching it). Situations you may end up in are much more complex.
edit: there's also L3, which would be more accessible. There plenty of ways, G1 was just too lazy or just underrated console needs (which they already proved to do).
I hope little orbit fixes everything that G1 thought we didn't need/were too naive to ask for.