#2. To put over Becky Lynch more as a babyface
Becky Lynch has now singled herself out as the only babyface in this rivalry. She was a heel briefly last year, but her popularity skyrocketed to the extent that she became the hottest superstar on the roster, with WWE forced to quietly turn her babyface again.
Ever since then, she lost the SmackDown Women's title (and the rematch), but it was a step up for her, as she won the Royal Rumble match and now has the opportunity to secure her spot at WrestleMania 35.
Naturally, Becky will win and go on to take her spot in the main event. Given that she's the hottest thing going in the women's division, Rousey's heel turn may have been done to further put over Becky Lynch as an underdog.
You can believe that Rousey will have no problem in taking the fall for Lynch at WrestleMania 35.