let me share how I use the trackpads: in most games, I customize the left one to behave as the left joystick, like really detailed custom controls, and then the left joystick becomes my shortcut hub: layers, action wheel, you name it.

The right track pad is straight up mouse, I customize sensitivity and pair it with gyro. Let's use deadlock as example, this level of customization allows me to play such fast paced shooter easily from my couch. Last year I used AI ( don't shoot) to build a small tool that in a nutshell turns the steam deck in a steam controller when connected via USB to my living room PC, now that the control is almost here I don't need the workaround anymore. i use the track pads primarily and the joysticks as secondary controllers

Essentially it's a controller designed to get a decent to excellent result playing anything on steam, part of that is you need a mouse and keyboard. I've used the ones on my steam deck a bunch, and they're pretty good at most anything.

There are lots of reasons. The primary one is for games that simply don't support controllers. Think Anno or LoL or Civ.

I wouldn't even say they're good for FPS. Mouse is still gonna be king for that.

They're also infinitely programmable and devs can even use them like any other input, like a weapon or inventory wheels.

I've found virtual/radial menus to be a nice use. In games that have more keyboard bindings than controller options you can bind keys to a virtual menu. For example I could never abuse quicksave/quickload on a controller in New Vegas before.

Games that are in designed for only Mouse & Keyboard

But I mostly plan to use it to control as mouse keyboard on my desktop.

My PC is hooked to my TV. Using a mouse on a couch fukin sucks. A track ball is alright but it has to be wireless and you still need a keyboard. This solves both issues.