Time
Explanation
This comic takes a grand existential question -- "Why is there time?" -- and gives it an unexpectedly mundane, entertainment-based answer.
In the opening panel, a person asks God (depicted as a glowing yellow figure): "God, why is there time?" God's response is surprisingly practical: "Try watching a movie when there's no time. Everything happens all at once. There's no dramatic tension."
God elaborates further: "We tried a no-time universe, but it made things weird. The sex scenes are right on top of the visits to a dying grandma, and the emotional breakup and the fights with zombies or whatever." In other words, without the sequential ordering that time provides, all events in a narrative (and in life) would occur simultaneously, making everything an incomprehensible jumble.
The person, stunned, asks: "Wait, time exists to make movies better?" God replies simply: "Also comic books."
The humor comes from reducing one of the deepest metaphysical questions in philosophy and physics to a matter of narrative convenience. Instead of time being a fundamental feature of reality for profound cosmological reasons, God invented it because stories don't work without sequential ordering. The "also comic books" punchline is a self-referential joke, since SMBC is itself a comic that relies on the sequential panel format -- which only works because of time. The comic playfully suggests that the entire structure of the universe exists to serve storytelling.