What evolutionary advantage can there be in humans having the hair on their heads grow indefinitely?

Hair on other parts of our bodies stops at a certain length. But, short of things like baldness, if we don’t cut the hair on our heads it will just keep on merrily getting longer until we eventually die. (No doubt due to tripping over it and falling in front of a car or something.)

Apes don’t go to the stylist for regular haircuts and the hair on their heads doesn’t do this – it grows to a certain length, then stops. Since we “ascended” from the apes, we’ve gradually lost the hair on our bodies that they still have. Yet, for some reason, our scalps have rebelled against this trend and gone the other way.

Any ideas?