You don't have to give me your email address, but I recommend you to do it anyway. I promise not to spam you.
If you enter an email address, the server will send you one mail asking to confirm that you actually own that email address. Only if you click the confirmation link, the server will be able to email you.
By default, the server sends you email on various events: your turn files, answers to forum topics you wrote in, and private messages. You can configure how many of these messages you want to receive, or turn them off altogether. Of course, when you decide to receive one type of message, the server must know your email address.
But if you're really sure that you don't ever want any mail from PlanetsCentral, just leave the email address field blank.
Note that password recovery requires working email. If you forget your password, you can recover your account only if you have entered an email address, and have clicked the confirmation link.
