Mainly for instances that are constantly at the 80 player cap, sometimes crashes happen and people get disconnected. It can be frustrating for event goers to lose their spot in a world from something they couldn't control.
If there was a grace period for users who are disconnected unexpectedly (Not leaving instance regularly or losing connection) to rejoin within like, 5 minutes or something, I think that would alleviate a lot of stressors in regards to general event experiences.