I'm sorry, I'm a web developer and I don't see the point in this either. Chrome and Firefox have mobile browser simulation features already and I can test my sites in more than one screen size using them.
I am a web developer too, and i can't see any point of this. If i would realy wanna test webapps or responsive websites, i would just do it in a emulator or in my realy phone.
I'm fairly certain this is more for users and not developers. Giving people an easy small window for little-apps makes a lot of sense to me.
It doesn't, is just a waste of resources for something that you could easily have just opening a new tab.
What tabs wouldn't provide you is the separation of resource management. Your little tabs would die, refresh, reload with the rest of your main browser (there may have other limitations, e.g. cookie size limit). Is that a good thing? Not always, at all time, at least.