I've absolutely loved being able to check a store listing page and immediately know if a game will run well on the Steam Deck. Having the same program for a higher end target would be really nice for me.
Also, getting the CEC right is really valuable for me. If I'm building a computer there's no chance I am going to be able to get it to play nice with the TV using just the controller.
There's also potential for community fixes for older games with issues. And easier troubleshooting cause you can just look up "fix for X game Steam Machine", or "does X game work on Steam Machine"
There are advantages to this over something generic, or building your own machine.