benefits? like i said - a lot of other things to consider
nighttime at movie world can have parades, WNW has movies, Sea World has light shows in the lake - you could offer these on alternate days to encourage repeat visitation (I know we hopped between DLC and DCA on alternate days to catch the early opens \ late closes \ alternating nighttime entertainment)
In our summer, many folks would prefer the cooler evening to visit rather than the heat of the day. it potentially captures people who wouldn't have otherwise visited.
(I know we personally would visit the parks during summer if they had late opens - as a matter of fact the last time i visited between Xmas and Australia Day was for a WnW movie night)
You can sell flashy crap because its dark.
If you lose the second day visitor to the night time visit, then thats capacity for the park on other days, but you haven't 'lost' anything if they've done two in one day and bought meals at both.
This isn't adding labour, work hours or anything else other than minor considerations for nighttime operations, its just moving the workday to a different slot. Doesn't have to be permanent, and every park can share the load and take turns on it. Doesn't even have to be every day - can just be 'select days'..
Finally
because why not?