There are several benefits to signing up.
I know some people probably wonder why this site even exists. The answer is simple - to amuse.
You can either write encrypted messages into your own notebook (you have to sign up for free to get your notebook) or you can write notes into the "public" notebook. In either case, writing an encrypted message is as simple as clicking "Write It" and filling in the big text field. Then enter a secret key that will be used to encrypt your message. That's it - your message will be encrypted and posted on the site and you can click on it to get a permalink
Alright, you got me. The key is stored on the server - but in an irreversible format. The key you use is stored using a sha1 along with a custom hashing algorithm of my own - in short, don't worry. Even if the whole database was comprimised - your encrypted message would still be as safe as it was before (which is pretty darn safe). The reason the key is stored in this format is to provide statistics to registered users. This way they can see how many times their notes have been successfully decrypted.