#2. To justify his heel turn

When a superstar turns heel, it's usually followed by a major push. Simply put, WWE needed to justify AJ Styles' heel turn and in kayfabe, it ended up working out because it resulted in a title.
Ultimately, Gallows & Anderson helping him meant it was a shortcut, but would the heel turn be as impactful had AJ Styles ended up losing to Ricochet? It reminds us of last year, when Becky Lynch turned heel against Charlotte Flair. It was instantly justified with her beating Charlotte Flair the following PPV for the championship.
Cody Rhodes' uncle is a WWE Hall of Famer. More details HERE.
Either way, now that AJ Styles has turned heel and become champion, it'll be interesting to see how he carries it around. One thing we don't expect from the reign, however, is for it to be a long one. As mentioned, the United States title's recent history suggests that he won't hold it for long.