The correct answer to that specific question: as a sane default, all sound should now go through the headphones. That's a sane default on the Iphone, probably on Macbooks, and it also happens to be the one that at least a stock Gnome or Ubuntu desktop chooses. In fact I have trouble thinking of a saner default than that one!
Forgetting about sane defaults seems to be a common problem in FOSS communication. It's unfortunate, because describing something as unknowable can convince a new developer to avoid inspecting the code around that particular problem.
E.g., it took a long time for Gnome to get the sane default I describe above. I'd bet that lurking in the appropriate mailing list history is at least one didactic post from an audio developer explaining how we can never truly know what a user's preferred setup could be.
That is the regular desktop user. If you do any for of audio processing (pod casting, music recording) you may have a large number of different inputs and output connected, and want to do mixing of some sort. Likewise if you do videos you may have more than one "webcam" that you want to mix. These are fairly common use cases (many people have a dream of being a musician or an actor, and so have this type of setup that they are trying to use in the efforts to make it big)