What is it used for exactly on the 21"? At that size?
1) Front facing Logo.
2) IR receiver.
3) Ledge for edge to edge glass/display to rest on.
4) Allows for internal space to be larger (something very much lacking on the 21.5" model).
5) Aesthetically pleasing.
"There is no rule that says the best phones must have the largest screen." ~RoundaboutNow