Why, in 2011, are people still selling books with hundreds of pages of reference material in them? Has anyone needed or wanted to consult a printed reference on the Python standard library in the last decade?
Having a book is like having an additional monitor full of information that is uncompromising on your task. It's also wonderful to read, and easy to pop open even when your laptop isn't out, and you're just curious "I wonder if Array has a method to do ...."
A final note, because of that, having a paper reference (especially an annotated one) lends itself strongly to actually reading large swaths of it, instead of just finding the method you need and putting it away. With it being clunky, you don't want to do a lookup twice, so you open it and keep it open, and often read through a module/class' entire method list. That really improves your knowledge.
So in short, because it encourages good work and work practices.