Mainly to pick songs that fit the mood of the audience. At the moment, humans seem to do a better job "reading" the emotions of other humans in this kind of group setting than computers do, and people are willing to pay for experts who have that skill.

An ML model could probably do a good job at selecting tunes of a particular genre that fit into a pre-defined "journey" that the promoter is trying to construct, so I could see a role for "AI DJs" in the future, especially for low budget parties during unpopular timeslots like first day of a festival while people are still arriving and the crew is still setting up. Some of that is already done by just chucking a smart playlist on shuffle. But then you also have up-and-comer or hobbyist DJs who will play for free in those slots, so maybe there's not really a need for a smarter computer to take over the job.

This whole thread started from the question of why a human should do something when a machine can do it better. And the answer is simple: because humans like to do stuff. It is not because humans doing stuff adds some kind of hand-wavey X factor that other humans intrinsically prefer.