#2. To further establish them as WWE's biggest "Power couple"

For years now, there's been only one real "power couple" in WWE - Stephanie McMahon and Triple H. That's something that's dated back to the Attitude Era, but times have changed. Stephanie McMahon is seemingly taking a backseat, only really appearing when required, while Triple H too, only really appears unless he's in a storyline or he's furthering another storyline.
That's good because that's the whole purpose of authority figures anyway. When WWE reposted Seth Rollins' Instagram post, the two were declared as WWE's newest "Power Couple", while fans jokingly called them "The Men".
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It's ultimately a good thing and if they are together for a long time and they could achieve a lot of things together. With Becky Lynch clearly being the face of the women's division and Rollins being the face of RAW, the red brand could become their show.
Also read: 5 big mistakes WWE made at Stomping Grounds 2019