This could address:
signing up for notifications and forgetting about it
not knowing how to turn off notifications
someone else using your email address to create an account
joe jobs (forging your @lj email alias to send out spam)
email linking to spam pages on LJ, not sent through LJ or with forged headers.
receiving spam through your @lj email alias
receiving spam through the email address you have displayed on your profile
This would be really helpful whenever news posts go out and we get a stream of "unsubscribe me" requests. Also, it's a good place to document http://www.livejournal.com/settings/?c=MangleEmail.