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.

Depends on the game. Most common setup is just mapping the ABXY buttons to them, so I don’t have to take my finger off the right thumb stick to press them. In Elden Ring I use them for dpad presses instead, because I need to cycle spells/use recovery items without stopping moving.

I know a lot of people use them for L3 R3 instead of clicking the sticks.

In a couple games with smaller text I use one for toggling the magnifier, or in keyboard heavy games for opening the keyboard.

I actually bind the top two as clones of the bumpers, Maybe I just have small hands and that’s why I’ve always liked PlayStation controllers more? But the bumpers on the Deck feel like a reach and not somewhere to comfortably rest your fingers on.

For the other two it’s normally duck & jump, or some button you have to regularly hold down while doing other things.

Honestly, I don’t. They don’t do anything and I only click them while bored on waiting screens.

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.

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.