From what I've read, although I can't remember where, the carry handle is a throwback to the AR-15's ancestor, the AR-10. The first AR-10's were manufactured with the charging handle running through a slot on the top side of the upper receiver. The handle itself sort of looked like another trigger. Anyway, in order to protect the charging handle from damage, and to overcome the problem that the typical space most rifles use for iron sights was now occupied by an important control,(the charging handle) what we now know as the "carry handle" was added as a platform for iron sights. Later models of the AR-10 saw the charging handle moved to the spot behind the carry handle that we're all familiar with, but the carry handle remained, and was passed on as the AR-10 design evolved into the AR-15. At some point, somebody figured out that the carry handle wasn't needed anymore, and flat-top versions of the rifle started popping up for those who wanted their scopes closer to the bore line, and the same happened in the AR-10 design as well.