Why does hair grow in your armpits?

In our evolutionary past the sense of smell was much more important than it is today. The hair under the arms and around the groin traps body odour and so intensifies your smell. It is thought that smell used to play (and still does in some respects) an important part in choosing partners. So having a stronger smell made you more noticable.

It is only quite recently that we have been worried about covering up our natural smell and replacing it with perfumes or colognes.