Why Does Galactus Eat Planets In The Marvel Universe?

Galactus is one of the most intimidating forces in the Marvel Universe as the cosmic juggernaut is specifically designed for a significant reason: To eat planets and maintain balance in the universe. But while he often comes across as a villain, Galactus' actions aren't driven by evil. Instead, he knows he needs to survive due to his otherworldly purpose.

Debuting in "Fantastic Four" #48 (by Jack Kirby, Stan Lee, Joe Sinnott, and Artie Simek), Galactus' purpose is revealed from the get-go, as he consumes energies of entire worlds to maintain his hunger — but not for his own enjoyment or to cause harm to anyone directly. Wielding the great Power Cosmic and driving a worldship across space, Galactus proves to be a powerhouse like few in the Marvel Universe have ever faced. While the Fantastic Four successfully prevent Galactus from eating their world, the Devourer of Worlds has continued to live up to his name, using his advanced technology and powers and a herald to find other planets to consume without much thought about who he kills or what he destroys. 

Ultimately, Galactus is actually a necessary component to the continuation of the Marvel Universe. His world-eating helps keep existence stable, bringing balance to life and death. 

In "Super-Villain Classics" #1 (by Jack Kirby, Stan Lee, Vince Colletta, George Klein, and Jack Morelli), readers learn that before becoming Galactus, the cosmic entity was a humanoid-like creature named Galan from the planet Taa. Sadly, his technologically advanced world is destroyed, with the Sixth Cosmos coming to an end before the birth of a new universe. However, before dying with his people, Galan is contacted by the personification of the Multiverse, who merges his essence with him to be reborn in the Seventh Cosmos after an incubation process, leading to the creation of Galactus.

A new wrinkle was added to the origin of Galactus in "Thor" and "Silver Surfer Black" (by Donny Cates, Tradd Moore, Nic Klein, Matthew Wilson, Dave Stewart, Joe Sabino, and Clayton Cowles) as a universe-devouring being named the Black Winter is revealed to be responsible for destroying the Sixth Cosmos. According to the formless entity, the Black Winter saved Galan from the destroyed universe in exchange for serving him. However, upon being put in incubation by the sentience of the Sixth Cosmos, Galactus never fulfills the deal.

Regardless of the more recent retcon, Galactus' origin dates back to the Sixth Cosmos of the Marvel Universe. He is around during the past universe's destruction, arriving in the new world as an important cosmic force.

Galactus has also taken on a stark opposite role from his usual role planet-eating self, once becoming Galactus the Lifebringer in "Ultimates" (by Al Ewing, Kenneth Rocafort, Dan Brown, and VC's Joe Sabino).

Following the recreation of the Marvel Universe after the events of "Secret Wars," some of Marvel's greatest cosmic minds — including Blue Marvel, Captain Marvel, and America Chavez — use the same incubator machine Galactus utilized initially to transform from his humanoid form to a Devourer of Worlds, changing him into something new. As the Lifebringer, Galactus does exactly what his name implies: restore and bring life to previously destroyed planets — even ones he once consumed. In the process, he begins atoning for his actions with each life he saves.

While Galactus succeeds in his new role, some of his fellow god-like beings question his place in the cosmic hierarchy of the universe. But Galactus continues working alongside the Ultimates and, later, the Eternity Watch, consisting of several cosmic allies, in taking on the agents of the First Firmament in efforts to take over the Multiverse.

Unfortunately for Galactus, he couldn't escape his past. He returns to his world-devouring form after helping prevent Ultron from spreading a deadly virus across space and time, destroying the planet Saiph, where the villain created an army of Ultrons, and restarting his hunger. But for a brief time, Galactus was one of the most important heroes in the Multiverse.