Aren't you allowed to hang out in the Plaza Garden for a bit after the fireworks? I believe I've read that people hang out there for a little bit until the crowds go down on Main Street and then they leave.
I can see the appeal in an after party. I can see people wanting to ride rides and take advantage of "lower crowds" while most people are getting spots for HEA. So this way you show up to watch the fireworks 15 minutes before show time and then eat after while the masses leave instead of showing up for the preshow at 7:30, eating and then watching fireworks at 9 pm. I would probably do the after party if we weren't doing it on arrival day and will probably be extremely tired by the time fireworks are done.