I enabled FileVault 2 on my Mac, and I remember that it gave me a recovery key. When will I use this recovery key?
You will need the recovery key to decrypt the volume if you lose your password or, in the unlikely event that the encryption key associated with the password becomes corrupt. If you elected not to allow Apple to store the recovery key, keep a copy (or several) in a safe place other than your computer.
Recovery Keys can also be used by System Administrators to mount volumes for maintenance or recovery. (The Admin doesn't need access to an account password). It is possible to encrypt a FileVault volume using a master key.
Apple's Best Practices for Deploying FileVault 2 white paper provides more information about the inner-workings.