When you finish the boss, you get God Options, which let you control physics and spawn crap. You need it to do the Grind Star achievement, which otherwise is nearly impossible to do.
I just tried to play the Demo again, found out the Demo won't let you load games. Although what really annoyed me was discovering Save and Exit wasn't saving my games. I thought they would be saved so I could load them in the full game should I somehow get it. :( So I lost that five star circle, then.