How is this useful?
Has it ever happened that after you've signed up for a few new sites you suddenly notice an increase in the (largely) unsolicited and unwanted emails you get (though you may be wary of classifying these emails as spam)? Sometimes it is hard to make out which site actually sent out the email that you're now having to clean out of your Inbox. This feature helps you know which site sent the email.
For example, if you sign up for site abc.com with the email address firstname.lastname@example.org, and for site def.com with the email address email@example.com, then you can instantly make out (by looking at the "To:" or "Cc:" field) which site sent the unwelcome email. Furthermore, you can use this differentiation in the "To:" (or "Cc:") field to create very specific filters.
Sadly, this system is not perfect. There are two situations that I know of where this can fail:
- If the site you are signing up for doesn't allow a "+" symbol in the email address. Though legal, some sites don't allow such email addresses (I've faced this personally).
- If the site sending the email doesn't put your email address in the "To:" (or "Cc:") field, but puts it in the "Bcc:" field, then this nice feature fails too. However, this doesn't seem to happen too often.
If you can think of (or know of) any other situations where this feature may fail, please let us all know via a comment.
Despite its lack of perfection (for reasons beyond GMail's control), I find this feature very useful—I hope you'll find it useful too!