#1 Allows The Fiend to avoid a loss
It's no secret that WWE is going to avoid The Fiend taking a loss between now and WrestleMania, but how they do it is ultimately important too. For example, just having The Fiend steamroll over Daniel Bryan at The Royal Rumble would do no good for Bryan's character, which is why the company needs another way out this time.
With that being said and The Miz now on the verge of a descent into madness, it makes sense for the company to use this in order to explain an interference during the match. At least that way, The Miz can continue to stay relevant, WWE can continue the recurring trope of The Fiend changing people and they can protect both Superstars.
Beyond all that, this outcome gives them a variety of booking options going forward and might even lead to a better matchup at Elimination Chamber. Either way, WWE needs to protect The Fiend, but doing it with an outright win seems boring. That's why an interference by The Miz makes sense and keeps the storyline going in a unique way.