At the moment there is no extra functionality if you do enter your email address but soon I'll sort a forgotten password form out and may add some email notification features in the future. You won't be getting any spam from me and if I do ever send out an email to all users it will only be regarding this site.
The forum ID field is also optional. To enter this, go to the memberlist on our forum and select your user profile. The page address will end with /member.php?u=xx where the xx is your user ID. If you specify a user ID then a link will be available with your replays making possible for other forum members to send you a private message.