Originally Posted by Ice_Dragon

Why? Because I say so of course.

And if you want the real answer, its to ensure that the competition is fair for everyone. If someone posts an entry, then realizes there is a big problem with it (eg. They realize something in the sig detracts from its appearance), or they get it criticized, and improve it, then they automatically gain an advantage over their opponents because they can improve their signature, and they can do this a near infinite number of times. It also means that when it comes too voting, everyone is voting the same sig, as it cannot be changed during the voting.

Happy?