Earth is a world populated by a wide variety of native organic lifeforms, most notably humans.

Apollo17 earth

Earth from outerspace

It is also a world that formed around Unicron who was defeated a millennia ago. It is also where vast deposits of Energon are located. Autobots and Decepticons have recently fought battles on this world. Surprisingly, Unicron is the core of the Earth and his blood flows out as magma.

After Megatron came for the Allspark, Earth became a planet of the Next Chapter of the War against the Decepticon forces. But that is the place and planet which was where the Battle and war of Transformers Legends began.