#1 It redeems AJ Styles as the main event talent
As aforementioned, despite a year-long WWE title reign, AJ Styles has barely, if at all, sniffed the main event. It's a testament to his talent that he is still over, much like a slew of world champions across the decade treated secondary to the favored opposition. He has the championship, but he doesn't feel like champion material even then.
By giving AJ Styles the main event against Brock Lesnar, it would solidify his status as the Alpha of Smackdown in a finer way than the reign itself.
A top AEW star just called one of his colleagues 'spoiled' and 'narcissistic' HERE.
In their first match last year, Styles took Lesnar to the limit. In fact, the notoriously lazy Lesnar gave a great contest, while also selling his leg injury even when the cameras weren't rolling. It shows that The Beast respects The Phenomenal One and might not mind doing the job for him.
Finally, with seven brand warfare matches set, WWE could place the two brands on a draw ending it with this thrilling champion versus champion bout as the decider. If Styles wins, it would, in fact, be the great underdog story as the face of Smackdown takes the perceived B Show over the line.
If their previous match was Rocky, then this one could be Rocky II as AJ Styles finally conquers The Beast. That's why it'd be the perfect main event!