Suppose corporate needs your bank's routing and account numbers to get you set up for direct deposits. Or the IT department needs your password to upgrade or modify your account.
That's not the kind of information you typically want to share via email. Or instant message. Or even fax. Ideally, it's the kind of thing you'd write on a sticky note and hand directly to the person who needs it.
Of course, that's not always an option, meaning you need some kind of safe, electronic way to transmit sensitive information.