As a tutorial for siege. NPCs would need to use player-like behavior repairing catas/archery towers, coordinating cata shots to insta-drunk stuff without it being repaired etc. Could be a way to make intermediate groups feel more confident regarding PVP, too.
Could also be a good testing ground for players to give feedback on siege. I assume if it were a regular thing people interacted with stuff like randomized ranges / damage would make ppl screech.
Bonus points if they NPCs could try to run away with some of the treasure in a way similar to players trying to get speedbuffs and click critters, but I assume that's unreasonably heavy programming for a tutorial
P.S: It's cringe that the queen in ant dungeons isn't getting fueled by her workers as a combat mediation tutorial