The want is external, stated, and drives the plot: get the job, win the case, reach the city. The need is internal, usually unadmitted, and drives the meaning: stop lying, ask for help, forgive someone. A story with a want and no need is a competent chase. A story with a need and no want is a mood piece nobody can follow.
The engine is the incompatibility. When getting the want would prevent meeting the need, every scene has two things at stake at once, and the climax becomes a real decision rather than an outcome. Make them compatible and the ending is arithmetic.
The commercial application is direct. A brief that says the audience "wants a faster workflow" has a want. The need underneath it is usually something like not being the person who is always behind, and that is the thing the film should be about, even when the product only addresses the want.
What is the difference between want and need in a story?
The want is the external, stated goal that drives the plot. The need is the internal, usually unadmitted thing that would actually resolve the character’s situation. The story works when pursuing one obstructs the other.
Why does a story feel hollow when the character gets what they wanted?
Usually because the want and the need were compatible, so the ending was arithmetic rather than a decision. The audience feels the absence of a cost even when they cannot name it.