A reverse email search allows you to find people using only their email address. With nothing more than a single email address you can find out a whole host of information, including: what social accounts they have; what their REAL name is; their physical address, even their phone number. In fact, many times you can even see who their relatives are!
In many cases, you can even see the IP address and location information about where the email was sent from. This is great if you think you might be getting catfished or scammed by someone you met online.