Darkseid is not only of the most ominous villains in the DC Universe, but in comic book history. As a literal (New) God, this intergalactic despot has razed Earth on several occasions, with each time coming closer to achieving his true goal - completing the Anti-Life Equation. But just why does Darkseid need Earth to accomplish this?
Darkseid first appeared in a cameo in Superman's Pal, Jimmy Olsen #134 of all places. Created by the legendary Jack Kirby, Darkseid is a New God and ruler of the planet Apokolips, rivaled only by the benevolent forces of Highfather on New Genesis. Darkseid's true intention is to remove free will from the universe entirely, reshaping it in his twisted image, through the mysterious Anti-Life Equation. His quest for his unparalleled power leads him to Earth, where Darkseid contends that the secret of the Anti-Life Equation is buried somewhere deep within humanity's shared subconscious.
Darkseid actually gained control of the Anti-Life Equation during Final Crisis, where he unleashed it on the Internet, gaining control of millions. He also was able to extend his control to the various super-powered beings on Earth, turning many of them into Justifiers and Furies, before he was eventually stopped when Wonder Woman bound the Lasso of Truth to him. Interestingly enough, the Anti-Life Equation is also responsible for kickstarting the zombie apocalypse in the recent DCeased. Once again, Darkseid is using humans - this time Cyborg - to gain possession of the Anti-Life Equation. However, the Anti-Equation becomes corrupted and ultimately self-aware, zombifying any it comes in contact - including Darkseid itself - before spreading to Earth.
Since DC Rebirth, the resurrected Darkseid has played an increasingly large role in the affairs of the DC Universe. He first as an infant strapped to Batman in Dark Nights Metal (seriously) before rapidly aging into his former self. He has also recently recruited the likes of Starfire, Cyborg, and Azrael for his own purposes in Justice League Odyssey, but the arrival of Green Lantern Jessica Cruz has short-circuited his manipulations somewhat. Still, it seems likely Darkseid is still pursuing his original plan to eradicate free will.
Darkseid's quest for the Anti-Life Equation is central to his characterization. He's trying to erase the concept of free will. For any other villain, such a goal would have ultimately futile, but because of Darkseid's imposing design and truly Machievllian schemes, he's come extremely close to fulfilling his plans on a regular basis. While Darkseid's mysterious plans have currently lead him out into far-off reaches of the Ghost Sector, it almost certainly won't be long until his desire for the Anti-Life Equation leads him back to Earth.