Besides Switch-like form factor (although a bit chunkier) and Steam games on the go Steam Deck offers:
Insane mount of control custamizability: you can play games that are only for mouse and keyboard from the comfort of your couch, bed, any place really
Great 3rd party ecosystem: there is a shit ton of stuff for SteamOS that community prepared to be easy to setup like emulators, plugins, non-steam launchers etc. Discovering that can be fun on its own
Find your own use case, customize hardware and software as much as you like with only sky being the limit.