My daughter's preschool had a small graduation celebration. It really wasn't a big deal, just the kids doing some skits and stuff followed by a potluck dinner. It was fun.
Our kids preschool had a little "graduation ceremony".
They did their little skit and the they had all the art\projects they did up on the walls for parents to view.
The women that ran the the preschool did it because they enjoyed what they do, wanted to have a send off for the kids and also give the parents an opportunity to take pictures. That and collect all the stuff their child had worked on at the preschool.
There weren't any private parties associated with it nor have I ever heard of such. Sounds like a big city / high society thing to me.
With friends and family who live "In the big city", I've never heard of a private after party for preschool graduation before this thread.
I'd assume its "one of those moms" who read some crap in a magazine or is over doing it to build her own social circles. Normally, post preschool grad or kindergarten grad is play date material where the mom meets up with another mom she gets along with and tells the kids to go play together...preferably in the next room.