The fight is happening in order to give this manga an adequate ending. The Kaguya arc really killed the manga. This fight will save it from the bad fate that was hovering over this manga during kaguya arc.
That has already been announced, so it is rhetorical at this point.
Personally, though, I think Shippuden should end with the death of suckske, release of IT, and the obligatory remembrance. Time skip afterwards into part 3.
Sasuke beats Naruto but he doesn't kill him. He reaches a hand out to Naruto (who is flat out on the floor) Naruto takes it and he picks him up, the moon and sun seals interact and IT is broken. Sasuke reaches a compromise with Naruto and they find the happy medium between revolution and world peace.