This email service option was launched a few years ago and synced with Timeline in April. Any messages sent to that address show up in your 'message inbox.' The problem with this is that Facebook changed it virtually without telling a soul, making it your primary point of contact on Facebook.
So, if someone goes looking for your contact information expecting to find your @gmail.com or @yahoo.com account, they'll be out of luck. But there is a way to change it.
Just go into your settings, and click 'About' under your contact information. There, you can choose which email address you want to be seen by other users.