i don't have a source. all i have is a guess.
odor is created by bacteria acting on sweat.
armpit hair allows more air circulation, helping sweat to evaporate.
in addition, it acts sort of like a wick, to provide a path for some of the sweat to get away from the skin.
sure is far from perfect, but air is healthy for many things.
as an aside, the first time i noticed what hair did was when i bruised my knee pretty good, and they shaved it before draining the fluid out.
all of a sudden, my pants were up against my skin, rather than held just a little bit away.
over many years, i've had various parts shaved, due to some mistake or other.
having the hair sure is nice. ;-)