Why buy a paper book, when you could have a thousand digital ones on a Kindle?
A paper book is something that's absolutely yours — something neither publisher nor bookseller can take back from you, something unbound by End User License Agreements.
More than that, a paper book is an object that continues to develop, with dog-eared pages, coffee stains. A second-hand book is a story that comes wrapped in a narriative of its own.
Most importantly, a paper book is something that forms a bigger picture of your inner self, your personality, or simply a history of the twists and turns of your own curiosity, when it's placed on a shelf with others. (The ones that don't go on the shelf also help create that picture, by not being there.)