I've always been a M+KB player and especially for FPS games I've always found it impossible to aim with the sticks, so the trackpads really helped in this regard. Especially for older games that may not natively support joysticks it was a game changer (I recently played Riddick: Escape from Butcher Bay)

I played through whole Half-Life 2 on steam deck with aiming and shooting using right touch pad and it was alright. Strongly suspect though the game should have a support for it properly otherwise it feels janky in everything else I tried with it. No idea what's the use case for left pad though - I sometimes play with it during loading screens due to nice sound it makes, that's about it

Besides being a mouse, you can map it to a virtual menu with as many shortcuts as you want, the shortcuts being key presses/combos, allowing you to bind way more things than would otherwise fit on a controller. You can also set up mouse regions so that the trackpad only maps to a part of the screen and will instantly jump there, like for moving around an inventory window or minimap.

For games without controller support or where KB+M has an advantage, the trackpads are a game changer. Imagine a hotbar like in Minecraft or Terraria, but instead of having to go left or right one at a time with your bumpers, you could spawn a radial menu with the touchpad and instantly flick to "8" or "5" or "1" to select those slots.

Played Factorio for tens of hours using the Steam Deck touchpad

Best usecase for them (according to me) is top down strategy/base building games

Some people use them for radial menus, like selecting a weapon or pings or whatever

The touch pads work great for emulating a mouse in certain games. I've played many hours of Civ V and VI on the deck thanks to the touch pads. You can even configure them to act as a radial menu or touch menu where different areas of the touch pad emulate different keys.

In steam deck desktop mode, the touchpad-as-mouse makes it just bearable enough to use. I can't use desktop mode with a wireless Xbox controller with no touchpads. It's really the missing input type that makes the steam deck a complete tv-docked PC.

Can't wait to use it as a mouse for my Windows-based couch gaming PC.