Sometimes you want the same thing in multiple places to help usability.
Where you might think to look first isn't where someone else might look
first. It's pretty clear that some people are going to try to adjust the
sound volume on the thing making the sound (the individual app) and not the
mixer control on the panel.
However, PA is what makes that possible. Note that I said "the same thing"
in multiple places. The volume widget in the app should modify the volume
of the PA stream, and be reflected in the desktop mixer utility, and vice
versa. Without things like PA, this becomes much harder. It's not even
possible without a lot of crap being shoved into the drivers.