Obviously other than being able to then contact users... But even so, having it as an optional field would do that just fine.
Maybe you are thinking that you don't want to send mails to users who did not opt in to get messages, so giving them the opportunity to leave out their e-mail address will be the perfect way to opt in or out.
But the problem is that, as soon as you have created a site with accounts on it, you need to be able to contact each user for one special case: when the user forgets their password and/or user name.
Users do this all the time. They create an account, and even if they remember the user name because they use it consistently accross sites (and many users do not act this way), they forget the password. Then they want to reset their password. And please in a quick, automated fashion, without writing the admin a mail and waiting for him to take care of it.
So imagine that a user has an account, and forgets their password. If you have their e-mail, you can send the new password to it, problem solved. If you don't, the account is lost. You cannot ask a user who forgot their password to provide an address in their password recovery request, because then everybody can get a password for a known username sent to their own address.
If you are making a site where you outsource the user management completely (e.g. only accept logins via OpenID or similar), then the above does not apply. But once you do your own user management, the ability to contact a user at a known address is not optional.