#2 Chance to pass the torch
There are so many benefits to having The Undertaker face The Fiend, but one of the biggest ones is the fact that it could act as a passing of the torch moment. In fact, between The Undertaker's history at WrestleMania, and The Fiend needing a career-defining win to cement himself, the character absolutely needs this match.
Think about it! What would be bigger than The Fiend defeating The Undertaker at WrestleMania 36? The answer to that is absolutely nothing and everyone in The WWE Universe knows that. What they also know is that having The Fiend defeat The Undertaker would do a lot for the future of the character.
Sure, WWE can go with Roman Reigns or someone else defeating The Fiend at WrestleMania, but it's only going to take away one of the key parts of the character. Now that doesn't mean The Fiend's streak should last forever, but ending it in such a bitter sweet way is beyond distasteful.
If nothing else, The Undertaker is the defacto last test for The Fiend and his paranormal powers. He is the one person that The Fiend could beat clean and everyone's jaws would drop. That would prove The Fiend is for real. That would prove The Fiend is here to stay and that would finally prove that he is the new face of fear.