I mean, it’s fine to do so, as long as you have PC hardware that meets your needs. Valve would be fine with it too. As long as it can run Steam, all good. For Valve, I expect that the Steam Machine is to provide an easy-to-set-up option a la consoles that let them move into the living room for people who have an issue with that. If you can already use/configure a PC and have one, then that option is gonna work too.