#4. To make sure she comes out the winner

Think about it!
Ronda Rousey could be taking an extended hiatus from WWE after WrestleMania 35, but that doesn't necessarily mean she has to come out of the main event match as the loser! In fact, previous rumors have speculated that the original plan was for Rousey to retain at WrestleMania, which would kickstart a riot, and have her lose it the next night on Raw.
Cody Rhodes' uncle is a WWE Hall of Famer. More details HERE.
Interestingly enough, the original plan was supposedly for Charlotte to win the belts the next night on Raw and not Lynch. That apparently changed as Lynch's babyface run got hotter and hotter, thus forcing WWE to reconsider. With that in mind, why not do the original plan that WWE had and then make it where Lynch wins the belts the next night on Raw.
In the end, it allows for Rousey to come out as the winner of the match, gives WWE the positive publicity, helps with the contract negotiations with Rousey down the line and will ultimately lead to Lynch becoming the undisputed champion. It might not be the most popular plan and would come as a huge disappointment for fans at Mania, but it would be best for business in the long run!