I use it as a scroll wheel on my Steam Deck often. Scroll with the left, point with the right.

You can set it as a radial menu for various inputs. Quick save, quick load, map, inventory, decapitation finisher, etc.

I often go through the effort of mapping menus, macros, etc yo the left trackpad. It’s especially good for PC games with lots of binds.

To scroll in menus or desktop, sometimes virtual menus for games requiring more buttons

Some people use left-handed controller layouts.

Question, who uses the left trackpad and what for?

Every RetroDECK user

I use it in mouse and keyboard only games for rotary and square menus for things like hot bar keys or f1-f12 and the like. Really came in handy for finishing my first playthrough of Dyson Sphere Program.