Stop thinking like a man that sitting behind a desk with no pressure on him. Think about it from the standpoint of a woman who has been planning a wedding for months, has likely invested a lot of money, and has all her family and friends at the event likely at great costs on their own part. She's being told by someone she loves and trusts that she needs to sign the paper for the wedding to go through and he's promising her that it won't hurt her and is just a formality.
Maybe you wouldn't sign under those circumstances, but lots of women in that situation are going to opt with trying to keep the man they love and save themselves from embarrassment over doing the smart thing. You can say it's their choice, and it is, but there's no reason for those women to even be put in a situation where they have to make a choice under those circumstances. Again that's some scummy shit to do.
Yes, the can both sign it waaaay before, so if the man is the one that is insistent that it be signed, he should get it signed before the planning starts not wait until the day before. And it most definitely does favor the guy if the pressure of the situation makes the chick more likely to sign at that moment than she would have months in advance.