Why an author might use a flashback or a flashforward instead of telling a story in chronological order?

Authors don't always follow chronological order -- this is usually because going in a strict order would make the book boring. You want to start your story when the major action starts -- if the reader needs to know extra information later, you'd use a flashback to keep from having to start way back before the action. You might also use a flash forward to give the reader a "teaser" and show where the action is headed.