The white plastic iMacs from the PPC and early Intel era were about four times thicker than the current design. Back then they were putting optical drives and 3.5" hard drives and AC power supplies behind the display; now, it's too thin to fit an Ethernet port. The current set of tradeoffs really doesn't make sense to me, but if you accept the thin design as a constraint the continued existence of the chin is pretty obviously necessary; those older iMacs could have much more easily ditched the chin.