My main reason is the same as mindcrime's and daggersandcars's: I found that every now and then I'd buy books I'd forgotten I already had, and it was annoying.

My main reason is the same as mindcrime's and daggersandcars's: I found that every now and then I'd buy books I'd forgotten I already had, and it was annoying. At one point I thought there would be value in recording where in the house each book was, but it soon became obvious that there wasn't so I stopped. If space considerations required some of them to be in boxes, it would become valuable again. (I do tag each book with what it's about, and shelve things by subject, so in some ambiguous cases the catalogue might be useful for finding books. Hardly ever has been, though.) It's occasionally useful when I know there was a book with such-and-such in the title but can't remember the author or the exact title (though not very useful since usually in such cases it's also possible to find the relevant shelf and browse until I find it. But you can't grep a bookcase.) And if I didn't have a list I'd wonder every now and then how many books are actually in the house, and waste time estimating. As mindcrime says, this isn't something that has much value for a few bookcases of books, but the value increases as the number of books grows.