So far the only games I've ever played on Deck that require them would be Portal 2 alongside the Portal 2 mod Revolution (or something along those lines). I think jump and maybe 1 other function is tied to the back buttons. Otherwise, I haven't messed around with controls enough to find a use for them.

flappy bappy wise the bottom ones are the joystick clicks, and the top ones are the shoulder buttons (i find my shoulder button pressed to be on the runliable side while using triggers)

In games where I've have to hold a face button, yet use another I'll use the back buttons.

So I mirror ABXY in Hades II for example, and charge up my B spell while I dash away with A. Much more comfortable to charge with the back B and dash with the face A button.

Fast forward in emulators. Also, the menu toggle in Retro Arch.

In Dungeons of Dredmor, an old mouse and KB roguelike, the community made a good layout where every button is mapped to a keyboard key to open the various inventory and crafting windows, etc. With the track pad right there to act as a mouse, it's honestly just as good as a mouse and KB. Maybe even better.