One good reason for this is that for things like credit card numbers, a good interface will accept leading, trailing, or interior spaces. Sites that make me type in values in exactly their own weird format are an abomination.
Site A: Ah ah ah! You put a slash in your date! No soup for you!
Site B: Ah ah ah! You didn't put a slash in your date! No soup for you!
Site A: Ah ah ah! You put a slash in your date! No soup for you!
Site B: Ah ah ah! You didn't put a slash in your date! No soup for you!