I actually read about this in some comment field somewhere else, but here is a great explanation of my intention:
I thought first that I could just e-mail to firstname.lastname@example.org and it would automagically come to that e-mail. That’s not the case. You still need to create a label and set up a filter. But still quite useable. In case for example you want to send notes to yourself or something like that.
Another cool thing, let’s say you register on a website. Then don’t register with your normal e-mail, register with email@example.com
Then all those e-mails from facebook will come to you – with the above address in the to: field.
Quite neat huh?
In case you want to find out who is selling your e-mail address you can set this up for each place you register and then when you get a spam message you can click on more details and see where it was sent to!