It's to verify that you're a real person --the same real person as attached to the bank account -- and that you're in the US (or whatever country). There are tax and liability considerations when you're moving money
Not to mention "Know Your Customer" and Anti-Money Laundering laws for money service businesses. Given that you can't really get "money" out of Mt.Gox at this point in time (only bitcoins), it seems like mostly a formality at this point so that next time the feds come to seize all of Mt.Gox's holdings, they can show that they've been crossing all of the t's and dotting all of the lower-case j's ever since the last time they unwillingly paid $5mil to the government.