I'm new to ICOs. I've bought into about 8 now and some release the coins right away, others I have to wait until the end of the ICO to get my coins, and ICOs I have to wait several weeks after the end of the ICO to get my coins (so they say).

Can someone please explain why they do that? Why not just give everyone the coins they buy right away, like some ICOs? And why does it take weeks after the end of an ICO to send coins? I don't get it.