I used to use this as a mild form of spam detection, like people do with "+" email aliases, by adding something to the "what do you want us to call you?" field that was specific to the site.

Then, if I started getting spam to that name, I knew who sold me out.

It was especially useful for physical spam mail that wasn't coming to an email address. I discovered quite quickly which magazine gave all my info to every company that wanted it.