On a membership site, sometimes you want to be able to access the email of the currently logged-in user, so you can pre-fill this information in a form.
You can access with using WebvantaScript as <w:user />, but this should only be done on a non-cached page, or you'll end up displaying one user's information to another user.
A better way to access user information is from the user cookie. Until today, that cookie did not include the log-in email address, but now it does.
Details are in the the article Accessing Current User Information.