#1. Makes Roman Reigns a heel character for first time in a while
Whether WWE likes it or not, Roman Reigns returning and attacking The Fiend at Summerslam basically makes him a heel at this point. Not only is that virtually unprecedented, especially since WWE usually touts Reigns as their biggest babyface, it also brings into doubt the future of The Fiend's tittle reign.
With that being said, Roman Reigns is pretty much universally accepted as a face, which makes him coming back and attacking The Fiend a troubling move. It's almost as if WWE just wants fans to forget about The Fiend and just be happy that Roman Reigns is back to take over the yard.
The problem with that however is that the company has spent so much time making The Fiend a babyface again that it's going to be hard to turn the needle. Either that or WWE intends to use Roman Reigns as a heel going forward, which could mean a lot of different things at this point in time.