IMHO, since not a lawyer. A preagreement allows for both parties to understand each others expectations. Often problems arise from clients, and service providers, when terms of their agreement are ambiguous. So if you have it writing, there should be little to argue about.
The way I understand it is that if they have it in advance then they also have enough time to understand it or consult with someone to help them understand it. Consequently, saying that they had to sign under duress (no time) while not understanding has less value legally.