#3 Saving his return to singles action for a bigger stage
I know that this is the reason why a lot of the WWE Universe does not enjoy Brock Lesnar's on-screen work. But the reason that he's seen as a special attraction is that he is one, and only wrestles on select occasions when the situation calls for it.
WWE could always have Roman Reigns return to singles action once again on a random episode of RAW. Alternatively, they can choose to wait until WrestleMania and have Reigns sell the kayfabe injury that he has suffered at the hands of Drew McIntyre on RAW.
A female WWE star said her life is in danger. Details HERE
By doing this, this non-title match for WrestleMania ends up becoming a special attraction. And isn't that the point of it all, to have a match that the crowd cannot wait for, because they've not seen Reigns compete in a singles environment in years?
You have your reason, WWE Universe!