Your story's ending is the final determination of whether the reader will go on to read more of your books or move on to another writer. The more compelling your ending, the more likely the reader will buy your next book.
It can be difficult to tie up all your loose threads if you can't remember them. Outlining can help you pace your ending and keep the event order straight as each unfolds. Think about your climax's impact and how everything needs to come together for a powerful ending.
Every writer's process is different. Some might start writing with the barest of structures, some might outline every point, and some might not apply structure at all until after their first draft. But it's a lot easier to write when you know where you're going.