At times people do not want to dual boot to windows. The idea of restarting a computer to another environment to run a process is a bit lanky in my opinion. A more convenient method is logging into an application and running it from there. It is the whole essence of virtualization
Another thing is that this is not limited to games but steam in this way acts as a task manager. Through steam I am able to stream programs such as IDE's straight to any x86 architecture in my network. That sounds pretty damn convenient as it gives me the ability to continue my work from any suitable platform in the house as long as it runs a steam program.
Even then, it's a convenience thing and I'd probably still make the argument that a dual boot would come in handy at times unless you're very certain you're only ever going to run Linux based software.
The whole streaming thing sounds a lot like a system to let people be lazy, which I'm not against or anything but it does sound largely pointless to me. It could be that it just doesn't bother me to reboot my PC or even move it to another room if desired.
Unless you're doing the whole multiple people gaming off one absurdly overpowered machine thing, anyway. Not sure what kind of performance you'd get out of that though with the higher end games coming out.