Themes are important because this is about story and story resonates with people and their lives, their journies, their experiences. Without a theme, how relatable are the characters and the plot - what makes it more than just people acting out some plot line - it's the theme that holds the story together and gives motivation to the characters. It is also what resonates with the reader - giving meaning to them beyond what happened in the story you have written. They can take it and apply it to their own lives and circumstances.

If structure is the house with floor, walls, and ceiling, theme is the ground the structure sits on. Audience may or may not be aware of theme. They may or may not perceive the theme the same as you intended, but they will walk away with a certain something haunting them, even if they don't know how to spell theem.