3 Allows WWE to move Rousey to Smackdown Live

Rumors have been circulating about Ronda Rousey moving to Smackdown Live later in the year, and the only way to really do that is to first have her lose the Raw Women's title. That way, Rousey is freed up from the Raw title picture and can then make the rumored move to Smackdown Live to face Lynch at WrestleMania 35.
Of course, WWE could always choose not to go through with the title change, but there are certain risks involved with doing that. The first is that it means less time for the company to establish a new star in Rousey's place on Raw, and the second is that it would almost certainly mean WWE isn't going through with Rousey versus Lynch at Mania.
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In the end, and from every imaginable angle, Rousey losing the title at The Royal Rumble just makes the most sense. Not only does it free her up from the Raw title picture, give Lynch a chance to get her heat back by screwing her out of the title and set up their match at WrestleMania 35, it also is an excuse to do the rumored move to Smackdown Live.