#1 It's the start of a tradition

Just as how The Demon King has become integral with SummerSlam, WWE is likely going to start making The Demon a regular feature of WrestleMania. If they book him well, and Balor keeps winning, then you can be assured that The Demon will appear at WrestleMania year after year.
Also read: 5 things WWE told us on the RAW before WrestleMania
Moreover, it makes sense for him to do so, because he only appears as The Demon once or twice a year at most. After all, it is a special attraction, and it's probably the biggest way Balor can draw in viewers. Balor already has something very special going for him at SummerSlam, and while he ideally shouldn't be defeated, WWE has a tremendous amount of potential in their hands.
Even if Balor were to turn heel, he could bring out a darker side of The Demon, something that he had done at NXT Takeover: The End, which also happened to be the only time The Demon ever lost in WWE.