Well if you are pre-ordering online it sends it to you as soon as possible its like you already bought it.
But in store its like they hold it for you. Its also a way for stores to get an idea on ordering them. So they can buy the amount of pre-orders and more.
So you dont have to go on midnight you could go the next day. But im sure some people just go for the fun of it and getting on it as soon as possible.