The 'carry handle' was also a fashion thing.
During WW2, infantry spent a lot of time marching on foot, everywhere. Most of that with rifle in hand or slung, sometimes while carrying other stuff. Usually not in a tactical manner, but a "slog from Point A to Point B for 30 miles along this road".
Stoner was a Marine and he'd spent his fair share of time doing so. Most other rifles designed during this time period either had a carry handle or something similar for the same reason. FAL being a good example. Mostly a designer's 'hey, why not?" going by previous experiences and depending on the army asking for it.
Since the AR was going to need higher sights than usual he combined the sights into that first to protect the charging handle, then, just to be useful. Even if practices changed and the 'bugout handle' was no longer used. Only time I've ever used it was carrying 12 M16's at once (three per hand and the rest slung all over me).
"And Shepherds we shall be. For thee, my Lord for thee. Power hath descended forth from thy hand, that our feet may swiftly carry out thy command. So we shall flow a river forth to thee and teeming with souls shall it ever be. In nomine Patris, et fili, et Spiritus Sancti. Amen."