A story can be written very easily. You just jot some words down and hope that they make sense. But to write a good story can be a little (or a lot) more difficult. A lot of times you want to have some sort of depth to your story that will stay with your characters forever. Something that they can take away from it.
For example, Fahrenheit 451 by Ray Bradbury has many themes, one of which is censorship. That, I feel is what the reader is going to take away from the story and keep with them in the long term. They may forget what Guy Montag was doing or even who he was or why he was doing things but if someone brings up Fahrenheit 451 they will likely remember “Oh, yeah! That was the story where firemen burn books.” And from there the censorship element is pretty clear.
In his novel, Bradbury created a way of demonstrating censorship and the overall ignorance of people by completely changing the job of firemen, leaving a lasting effect on the readers. This is something that people are going to be looking for.
Now, your demonstration of themes doesn’t have to be as extreme of an example but you definitely want to create something that your readers are going to remember especially if you’re writing a more serious story.