There's a feature that compares your library to that of other members, makes recommendations for books that you don't have (is that even possible for some of my blog audience???), allows you to write reviews and many other features.
Part of me suspects this is a "right/left-wing conspiracy" to collect information about me to be used during some future coup attempt and add me to an Enemy of the State list. But truly, when they look at my library, I'm sure they'll pass me buy. Just how threatening is a person reading "Leslie Linsley's Weekend Decorating" and "150 Ways to Play Solitare"?