For several reasons. Reading is great exercise for your mind, like doing push ups for your brain. Reading increases your vocabulary, making you more intelligent. Reading is more healthy than watching tv, it is also a great way to pass down information. Lots of things like history and science can be gained from reading.
You learn not just about the topic of the story, but you learn good grammar and spelling without realizing it. Like the difference between their, there, & they're , and then & than. A favorite quote of mine is "reading gives us a place to go when we have to stay where we are."
It all depends on the types of books. If its a educational book it's to get smarter. If its a fantasy book, for me it's because it makes me feel like you are part of this new life. That's not ever going to happen to you. It makes me feel like I have a better life, a life I wished for.