One of the most important things librarians do to manage their book collections is weed. That involves removing the really old, useless stuff to make way for the shiny, new, useful stuff. Shelf space is limited, of course, but you also don't want to clutter your shelves with items that are too outdated to be useful to your patrons.
Academic libraries have a slightly different mandate in this area than public libraries -- we often serve patron communities that study the history of various disciplines so for them the old stuff can also provide valuable insight into the history and culture of those disciplines.
All this to say, it's pretty amazing what you can find sitting on some dusty old library shelves.
The Awful Library Books blogs is unearthing some books that are overdue for weeding.
Don't forget to read some user-generated comments on Why We Weed.