Reigns vs Cena was supposed to be the main event of this show, with Summerslam being Brock vs Braun but a lack of faith in Braun's drawing power and a need to scrap a couple of matches from the card led to the fatal 4 way and now Braun's good showing has resulted in his getting his own match. It was supposed to be a one match show and Brock wasn't slated to defend the title again until TLC.
Cena is expected to vanish again in just over a month so, given that Reigns and Lesnar will be building to their big match by the time Cena returns, this was the only time to really put the match together. Add in the fact that they have a huge stack of talent now and there's really no need to hold back on dream matches because you know a new flavour of the month will appear and we can just move people like Nakamura and Roode over from SDLive in the post-Wrestlemania shakeup and have a whole load of new opponents for people anyway.
The result could go either way, though I expect Roman to win given how he was booked in that promo segment.