Umm, I thought they wanted you to at least open the email so the HTML viewer pulled ImageX or dataY from their servers which was setup as a trigger to show your email address was a valid one.....
All they need is one good hit from one of their image engines to show your email address has a user looking at content then BAM, they send you the really good stuff (IE "how to enlarge your <blank>", "Singles in your area" etc). The odds of you clicking on one of the other ads is low, but still they have a shot of 1 in 1000, so all they need is 1000 good email address that read their mail.
All they need is one good hit from one of their image engines to show your email address has a user looking at content then BAM, they send you the really good stuff (IE "how to enlarge your <blank>", "Singles in your area" etc). The odds of you clicking on one of the other ads is low, but still they have a shot of 1 in 1000, so all they need is 1000 good email address that read their mail.