A Jump-In Roleplay is like a gathering at a local party or club. You've gone there with some friends that you've called up and told them to come out. While you are there some other friends, or friends of friends happen to be there and it becomes a sort of mix of a few different social group. You may have been the one to suggest the venue but because you are there with so many people you're not really in charge of anything.

A Roleplay where you have to apply is more like a catered event with Free/Casual being a private function with some drinks and food, and Advanced being a 5 course meal with a musician playing in the corner of the room.

Now imagine opening the doors of a catered, 5 meal, private function, to the public and allowing everyone to join. It might go well, but there's always the chance that there is that one guy that can really screw things up, make a mess, has to be asked to leave, and then you have to clean up the mess he left.