Steve said
It's kinda like a legal thing to them.
Actually, it is a legal thing, in every sense of the word. Its what allows them to get away with all their wacky rules and methods of enforcing them. Once you are baptised, you have indicated that you are a member of the religion, and you agree to be bound by its rules and procedures. In fact, its why the two questions that baptismal candidates are asked were changed back in 1985-6 to make this even more evident. Of course, this is never really explained to prospective members.
No Apologies (of the baptised before 1985 class)