When writing my own stories, I must have a title to help me get started, even if I change it later on. I am very fond of "open" titles where I could go in several directions with it. It helps to keep the reader guessing!
With books, I want a title to intrigue me enough to make me want to go on to read the blurb and maybe then the first paragraph or two. I then want to read the book to see if it lives up to the promise given by that title and the blurb.
But I never buy a book on the strength of its title alone. I see the role of a title is to "get me through the door" so I look at the book in more detail in the first place.
Some of my favourite titles include The Lord of the Rings (don't you just want to know who the Lord is?), The ABC Murders (how can the alphabet be relevant to a crime), and The Lion, the Witch and the Wardrobe (got to find out where they all come into a story, yes?).