#2 It gives both men fresh feuds

One thing that has become abundantly clear over the last couple of months is that when Roman Reigns or Brock Lesnar is occupied, the WWE struggle to find something for Braun to do.
Strowman has been built like an absolute monster over the last year, and it stands to reason that his only legitimate competitors are fellow monsters. Because of this, Strowman stands the chance of getting lost in the shuffle after WrestleMania, especially if rumours are correct about Reigns feuding with Seth Rollins.
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Putting him in the tag team division not only keeps him on TV, but it keeps him relevant and lets him continue to develop his in-ring work while working with different wrestlers like Sheamus and Cesaro, or The Hardys.
Likewise, a run with the tag belts will give Elias his first legitimate storyline with the company. Elias has not had a feud of any real substance so far and has pretty much risen to popularity through his character work alone. Giving him a chance to show he can perform between the ropes, and work an extended program, will help elevate him up the card as well.