The reason is just that I want to go in surprised. Very often I've read books or watched movies that I probably wouldn't've because I thought "that story is dumb" or "that setting isn't for me". I don't want to read a book and think "When do we get there?". I've even disabled the progress display on my Kindle because I don't want to know if what I'm currently reading is still set-up or the grand finale. If I know a thriller has 100 pages left when the murder has been found I know that something is still going to happen and that affects how I read the rest.
It may have to do with how much you like being surprised and why you read a book. If it's for the prose then the surprise doesn't matter. If you enjoy a story well told then the surprise doesn't matter. But I like being surprised and going into it without any bias and as little prior knowledge as possible.