Why did Daemon bend the knee?

The entire season has been baiting us with the notion that Daemon (Matt Smith) will betray his wife (and the woman he groomed). Throughout his time at the haunted Harrenhal, he’s been plagued with twisted visions, including one where he makes love to his own mother and another where a young Rhaenyra (Millie Alcock) chides him for how he treated her. As the season has gone on, it’s been clear that his desire to wear the crown is dwindling, but it isn’t until this last episode that we’re certain where Daemon’s allegiances lie. His final vision is one that sees far into the future, so far that he sees Daenerys, and sees Aegon the Conqueror’s dream with his own eyes, validating Viserys’ decision to name Rhaenyra his heir. That vision convinces him to remain loyal.

In the book, Daemon fights loyally on Rhaenyra’s side in a hugely important battle, and it’s likely that the showrunners wanted to end up there—they just decided to remind us that men aren’t all that fond of women ascending thrones instead of them along the way.