http://www.straightdope.com/classics/a1_338.html
Since the original column in 1984, direct mail has undergone several revolutions, in particular the sale of lists and, of course, SPAM.
The name and address of someone who responds to direct (i.e., junk) mail is pure gold, for which buyers of addresses for junk mailing will pay a substantial premium. Heaven help you if you give they your email address, which will probably count as an opt-in to all possible lists and will bring you SPAM, SPAM, SPAM, SPAM, SPAM, SPAM, SPAM, SPAM, SPAMMITY SPAM, WONDERFUL SPAM.