Why would anyone buy a console? Plenty of reasons, pretty much the same reasons people buy consoles now: Console exclusive games for one. I also doubt they will be able to sell a steam machine for a console price that will be able to play games at console levels. Also the ability to play split screen on one machine, which most pc games wont do. As well as the ability to play online with friends who already have consoles.
Honestly, i dont think steam machines will change much. I am sure there will be steam fans and microsoft haters that will embrace it, but otherwise, it is just a gimped SFF pc that probably will not be any cheaper and no windows OS to do other tasks. I also expect DX 12 will negate any performance advantage Steam OS would have brought. If valve can bring out exclusives or convince devs to do so that would be a plus, but it seems like a small market, but valve has plenty of money to throw at them.