Imagine someone from a non-profit speaking to an audience of potential donors. She shows a slide of a distinguished businessman on a suit in a fancy office and says that this man was once homeless. People will listen.
But what if she started with a slide of the man while he was homeless? The lead-up to the ending it would be far less interesting. It may be a surprise ending, but it's a far less interesting story.
Knowing the ending does not answer all the questions—it poses more questions. Plus people understand why they are they are listening, or reading, or watching.