By accessing the material they might accidentally leak further information.
If I want to know your IP address and I have your email address, all I do is send you an email with an image in it or some other "click here" net thing. I tag the image or link with a unique ID. As soon as you load the image or click the link I now have your IP address in my server's logs. Or maybe I post a links on my Facebook friends' walls -- again with the unique ID. I might learn a bit about their friends who click on the link. Maybe I seed the pages that the links go to with other types of enticing links, pretty soon I can see that all of so-and-so's friends clicked on the "anal sex" link but ignored the "help starving children" appeal. I might even get some of them to leave comments on articles that reveal even more information, like correcting "mistakes" in the linked article or revealing their own email address to "sign in to comment" or whatever.
Before long I know enough to mount a successful social engineering attack on a group of interest to me, possibly netting even better access to more directly useful information.