Some functionality that keeps me using the Steam Controller has been touchpads for movement, quick gadget swapping with touchpads, quick 180s/gyro activation with touchpads.
I use it to play games like Doom Eternal, Left 4 Dead 2, Spin Rhythm XD, The Finals, etc.
Sadly I don’t find myself doing the same on the Steam Deck opting for the joysticks instead, since I don’t find the placement, smaller size, square versus circle, and flat surface versus concave offering the type of consistency that I use the touchpads for on the Steam Controller. It feels more a touchpad appropriate for slow paced games or just desktop navigation than optimal for the way I use the touchpads on the Steam Controller. And I don’t see Valve delivering on the touchpad end that I would like with the next Steam Controller with all the inputs it would be fitting in.
Not an issue for most players who will be using joysticks as their primary, but sucks for Steam Controller dual touchpad users who’ve been waiting years hoping for a Steam Controller with updated gyro, better bumpers, and 2 more back buttons.
I used left touchpad as a touch menu when I was using the left joystick. After moving to the left touchpad I prefer it over joystick due to being able to rely on a sprint hold mapping it on the outer edge without accidentally triggering it like I would on the joystick. And I’ve come to love mapping stuff like crouch, slide, or dash to it too so combining movement actions to touchpad clicks. Frees me to use back buttons for other stuff and further reduce my need to lift my thumbs away from movement and camera controls.
That’s my experience as left joystick and left touchpad user.