Earth is the home of the human race and it makes up the world that they live in as of now. Humans live and require certain things to live on Earth, like food, air and shelter, etc..
When Unicron was set adrift, ton of planetary debris gathered around his body and over time formed the planet called Earth and humanity came forth to inhabit the world.
Earth was seeded with Energon when Cybertron was dying. So much was present that Megatron sent a squadron of Predacons to defend his resources and kill any Autobots. The Predacons evetually died becoming part of human mythology. When he experimented with Space Bridges, Shockwave rediscovered Earth and harvested DNA from dinosaurs and merged it with CNA of the Lightning Strike Coalition Force to create the Dinobots. After the Exodus Alpha Trion launched relics to Earth knowing Optimus Prime would reach Earth one day.
Team Prime made its way to Earth and a message was sent out inviting other Autobots. Using information extarcted from Arcee and Cliffjumper, Starscream moved his ship the Nemesis to Earth for the Energon and to kill Optimus Prime. When he arrived he constructed a Space Bridge high in Earth's orbit for any interstellar needs, hoping he would never have to use it to bring Megatron back to the throne.
Both sides eventually discovered the reason Earth seemed so important in Cybertronian lore was that Unicron formed its very core and Earth was the richest planet in Dark Energon. Unicron threatened to destory Earth, but an unlikely alliance between Autobot and Decepticon forced him back to stasis.
The Decepticons eventually rebuilt the Lock and intended to cyberform Earth into New Cybertron and eradicate humanity but were stopped by the Autobots high above Earth when Megatron died and his corpse fell to the planet below.
Main Article: Earth Gallery