First, men often do it because in Mediterranean countries where it is not only exceptionally humid as well as hot, shaved pits mean less problems with the growth of smelly bacteria that clings to armpit hair (and crotch hair). It's most often a hygiene thing. As I'm sure you're aware, folks in the UK don't often romp about in nothing but shorts and muscle T's. Your part of the world is most often cold and damp. No offense, but I'm somewhat surprised that . . . for example the Scottish, they're as good as anyone to pick on . . . don't braid their armpit hair into bulky cardigans. But if you spend enough time in a climate where it is not uncommon for the weather to be 40 degrees C and 130% or higher humidity, you soon understand that showering four or more times a day is not enough to keep the funk away.
Many LPSG-ers moan and whine that they prefer a "full bush" on both men and women. But once you get into the habit of manscaping your arm pits and crotch area you'll soon understand the benefits, or at least the pros and cons (unless you're French).
Just kidding about the French. Honest.