A chin is the lowest part of the jaw on creatures with mouths. Transformers have had chins since as long as they've had faces. Why is something of a mystery, since Transformers do not technically need mouths (and some don't even have one). One must also ask why certain bots have such large chins when they don't actually need them. Humanization? Experiences with mammalian lifeforms? Required for socially acceptable chin-stroking when plotting? Only Primus knows the truth.
It is worth noting that prior to Animated, many of the Transformers had triangular or rectangular "beards". Megatron had originally lacked one but after being reformatted he too received a beard. This is most likely due to the Japanese origins of Transformers, which makes them fairly consistant with every other typical robo anime.