Earth is where most of Doom II takes place. In this game, Earth has been invaded by the forces of Hell and most of its population has been destroyed. The player battles the demon forces here in order to allow the survivors to escape the ruined planet. At the end of the game, the invasion is defeated, and the survivors can presumably resettle and start rebuiding the planet.
In Doom 3, Earth is never seen in the game itself, but is mentioned multiple times as being the center of human civilization, as well as being the ultimate target of Dr.Betruger and the demon forces. It is hinted that the Ancient Martians may have fled to Earth after the devastating war with Hell, and that humans may be their descendants. It is also mentioned that at some point in the past, the demons were actually on Earth but were defeated and driven into Hell, hence their desire to return and retake the planet.
See Earth (2016).