One use case is building a secure messaging system between sites ... something more secure than SSL.
I have used this before as student exercises
(Even got written up in the local paper for teaching them to be hackers, shame on me!)
Another use case is encrypting some critical licensing or permission stuff, which can only be decrypted using a (paid for) license key (aka decryption key or seed). I have had developers ask me how to do this.
Another use case is storing sensitive data in a database ... stuff like salaries, that you don't want just anyone to be able to see, without limiting access to their phone numbers etc. That could apply to an internal staff DB. And yes, you would normally keep salaries inside payroll, not a contact database. Just sayin'.
