When you write a story, do you know the ending from the very beginning, or does the ending take shape in your mind as the story progresses?
You shouldn't know how to start a story, if you don't know how to end it.

Although it may reshape or change entirely over the course of the story, you need to have a goal; a safe haven. If you don't have anything you are going towards, you will put your story at risk of losing its way, wearing out and overstaying its welcome. You will just end up meandering around, unsure of where to go, after having gone all this way without knowing where or when it would even end. Which also means you may be able to incorporate earlier setup into your finale, but ultimately, your destination will only really be hinted at when you are close to it already. And readers can tell.
Everything will just feel like an ass pull at the eleventh hour, unless you are really good.
Nobody tells you to plan - just know where you want to go, so you can subtly communicate it throughout the story.