Having an end in mind will help you to keep on the track, discarting things that doesn’t match your final story.
A clear picture of how your story ends is important to define your next actions.
As the chess grandmaster José Raúl Capablanca said once…
To succeed, start the endgame before everything else. For whereas the endings can be studied and mastered by themselves, the middle game and opening must be studied in relation to the end game.
Define the end of your story and after, only after, assign the next steps that will build it.