You make use of them, to remap any functionality that makes sense to you. I usually just remap 1-2 buttons to the back buttons though, but have used all 4. It's quite nice when you set it up and tends to free your thumb from moving between the touchpad/joystick and buttons all the time.

I do a lot of my gaming with the Xbox Elite Controller. So I tend to remap some buttons to the back triggers for easier access (particularly for FPS).

So the back triggers for me on the steam deck is for consistency, so I can easily switch between gaming with my elite controller and the steam deck with minimal differences.

The select button is small and hard to reach from left analog, so I plan to remap Dmc V taunt to L4 or 5. Haven't tried it though.

If the upcoming AC6 isn't being restricted with console joysticks, I guess the back buttons can be used as more inputs for weapons or extensions.