Depends on the game a lot.
Sometimes it’s just ABXY buttons where I don’t wanna move my thumb off the right joystick.
Sometimes, one paddle activates a layer shift to have more mappings.
Like if a game has more controls than you could fit, layers can help extend the possibilities and paddles area decent way of activating them.
If a game is heavy on QTEs, like spam X really fast to do something, I might just map a paddle to enable a layer shift that turbo spams the other button.

Clones of face buttons.

Discord Push-to-Talk.

In Factorio the keyboard modifiers (alt, ctrl) are back there.

Playing heaps of Kingdom Come: Deliverance at the moment, I have one set to toggle Up and B for autorun, and another to take a screenshot. I’m sure I’ll find more uses the longer I have it.

Modded Stardew Valley - mod keybinds are on back buttons and virtual menus.

Helldivers 2 - stim binded to a left back button.

Hades - binded R3 / Reload to a back button.