The site claims that by submitting your email address the amount of spam you receive will be reduced because it will take the email and have marketers remove your address from their lists.
It claims to be run by a marketing industry group that is tired of getting complaints about spam.
Oddly enough the site is done up in a way that mimics the FCC's do not call registry site and until recently did not have a warning that the site was not run by the FCC at all.