Actually that's not true. One way encryption is often for useful.
Say it's for passwords...
then someone submits a password, you encrypt it and store it.
Next time they submit the password, you encrypt and compare it to the stored encrypted version. If the encryptions match, it was the same password, if the encryptions do not match, it wasn't the same password. You have all the information you need... it's a simple true false. And this way if someone snags your password database, they still can't get in to your system so you have some extra security.
-Rob