Why would you use a controller with the steam deck? It is a controller.
Some people like a lighter weight and different grips. Lots of cases have a kickstand where you can prop up the deck and use your favorite controller.
Can’t it also dock to a TV or monitor?
Yes
If you have a dock. Maybe I’ve been unlucky but I don’t recommend the official dock. Every time I want to use the deck in a docked mode I have to rewire everything in the correct order to get the dock to output a normal resolution. Wire it in the wrong order and you either get no output or you get low resolution output with weird artifacts.
I have the thin silver JSAUX one and don’t have an issue like that, but sometimes it refuses to charge until I reseat the AC adapter.
I like the steam deck and use it as a controller, but you could make the same argument about the switch. Regarding the switch, my answer would be that while it is a controller, it is - IMHO - a poorly designed and uncomfortable-to-use controller.
While you and I like the ergonomics of the deck, others may not. One can hardly blame those people for wanting something they feel more usable.
