I think it's a place where bibliophiles can show off their libraries to other similarly minded folk. It's a kind of virtual bragging (ooh look, I've got a big one). But also it's been the only incentive I've had to catalogue the bastard thing. Mind you there are some private libraries on the site that have more than double the number of books that I own. So it's a place full of book lovers where I can just about fit in.
Sometime between now and the end of the year my task will be complete.

I wouldn't include all the books I've read as IO think it's just about what one owns.