When writing a short story what is the reason for using a story map or plot diagram?

If you don't have some way to keep track of what's going on, you forget what you have written! I keep a list of all my character names, all my locations, what has happened in each scene, and what is going to happen in the future. That way, I can move things and write the story I want to write, instead of bumbling about wondering what went wrong.