What do you use the back buttons for?

Some games have button layouts that make certain actions a pain. Two examples.

Horizon Zero Dawn - Healing is by default done by hitting dpad up. You generally want to press this button whenever you take damage to essentially trigger health regen, but doing so requires taking your thumb off the left stick, which means you can't simultaneously avoid even more damage. Bind to back button, problem solved.

BallisticNG - Weapons are bound to X, discard weapons is bound to B, and accelerate is on A. So when you pick up a weapon, to discard/use it you either have to drop thrust (bad, never do that) or awkwardly shimmy your thumb to either hit X or B without letting go of A. Bind X and B to back buttons, problems solved.