The Emperor’s insistence on being there to witness the Rebellion’s defeat is a form of unintentional self-sabotage.
He spent his whole life on this conquest for power, and when he finally gets it… there’s no thrill. He spends all day sitting in a chair. How boring is that? He needs to put himself in positions where he can get a new high off of showing off his power.
That’s why he needs to be there for the Battle of Endor. He needs that high, because otherwise he’s just feels empty. And it’s ultimately his undoing.
No, he’s there because he wants to ensure the Rebellion falls for his trap.
Mon Mothma said:
But most important of all, we’ve learned that the Emperor himself is personally overseeing the final stages of the construction of this Death Star.
His presence is the final piece of bait that draws the entire fleet into attacking immediately.