#1 Finally establishes Roman Reigns as the top guy

If nothing else, WWE has been wanting to make Roman Reigns their defacto top guy for quite a while now and there is no better place to do that than against Shane McMahon. At least that way, Reigns is up against the most hated heel in the company, which will almost definitely put the fans behind Reigns by default.
Beyond that, it would also serve as a christening moment for Roman Reigns, who has needed one for quite a while now and would probably be enough to keep Reigns in the title picture for the immediate future. Again, it's just the perfect scenario to create a star and WWE just needs to pull the trigger on it.
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In the end, Reigns's official christening has been long time coming and the WWE Universe seems more behind him than ever at this point. With that being said and how despised Shane McMahon is as a heel right now it only makes sense for WWE to pit Reigns and McMahon against each other to create a monumental moment.