Marvel Comic's he is a central component of the universe itself and all the worlds that he consumes are destined to become fuel to create the next universe. This does beg one question however-why would a cosmic god essential to the balance of reality look like a human wearing purple armor? The answer is simple- that's not what Galactus actually looks like.
As an abstract being, figuring out how the anatomy of Galactus works is a difficult task, but someone's got to do it. In Fantastic Four #262 writer John Byrne decided to finally explain the reason behind Galactus' humanoid design. The issue, titled "The Trial of Reed Richards" is about Mr. Fantastic and the rest of his family being arrested by the Shi'ar Empire for the crime of having previously saved Galactus' life in Fantastic Four #244. Since Galactus was feared throughout the cosmos for destroying planets, the Shi'ar blamed Mr. Fantastic for allowing Galactus to continue living.
Presiding over the trial were other cosmic beings including Thor's father Odin, Uatu the Watcher and even the writer John Byrne himself. Suddenly, in the middle of the trial, Galactus himself makes an appearance, and it's revealed that each of the dozens of alien races attending the trial perceived the devourer of worlds differently.
This reveal is brilliant because it helps emphasize the otherworldly nature of Galactus and how he is more than just an alien, but a godlike being and a sheer force of nature. The rest of the issue was dedicated to Galactus pleading his innocence and coming to the defense of Reed, defending him as a good and honorable man when Eternity, another abstract being and "brother" to Galactus is summoned to defend Galactus' right to exist.
Eternity decides to share the "cosmic truth" with the mortals gathered at the trial, and in one brief, moment every living being is made aware of the universal oneness of all beings. This moment is enough to convince the Shi'ar that Galactus' presence is crucial to the survival of the universe, and both he and Mr. Fantastic are both acquitted. Galactus may not have made his debut in the MCU yet but thanks to Eternity and Mr. Fantastic the people of the Marvel universe now understand how important Galactus is to maintaining the balance of the cosmos.
