For instance, if the father wasn't a custodial on the account he can't deposit money in the event he needs it. My parents would have to mail me a check or pay a $25 bank transfer fee because they bank elsewhere. With student privacy laws, the father can't directly see school bills. Those have to go to the student, even if the parents are paying. My school also had a student account where you could pay for food, vending, and laundry. This is a separate account that requires replenishing, so the guys I knew would sometimes call their folks asking them to deposit some money for expenditures (freshman/sophomore year, everyone pretty much had some side income by then).
When did that start? I've deposited money many times into other people's accounts without problems; you don't even have to show ID -- just fill out a deposit slip with their account number and hand over cash.
When I was younger I had a joint custodial account with my father, for a similar purpose, though it wasn't my primary checking account. It's much, much easier to move money between two accounts you control.
as caw[1] said, it's really more so that he can easily transfer money to the account in the event I need it.