#3 Credibility

One of the foremost issues that Roman Reigns is grappling with, as far as getting over with the fans is concerned, is credibility.
He looks the part of a top guy. He certainly works the part in the ring. But he doesn't feel the part as far as many of the fans are concerned.
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The WWE may build him up to make him seem like the top guy all they want but if that doesn't translate to the audience, it's all wasted effort.
That's where an opponent like John Cena kicks in.
He's a bonafide, top-of-the-line Superstar that is teeming with legitimacy. And although Roman has been feuding with Strowman for most of the summer, the scalps of The Undertaker and John Cena do make fantastic additions to his resume.
Yes, The Undertaker was a washed-up veteran by the time he wrestled Reigns, but John Cena can still absolutely go in the ring.
And a victory against him comes stapled together with bucket loads of credibility. Even if it doesn't straight up get the fans cheering for Reigns, it still gives the WWE a powerful marketing thrust to build a convincing narrative around.