Great writers are great at subtext. Great subtext makes you a great writer. Great screenplays are loaded with excellent subtext. There are different types of subtext.
Be able to create one kind of subtext on every page of your script. Why?
The answer is to think about how people experience your script.
Love subtext because it gives them the chance to act on multiple levels, saying one thing, but delivering a deeper meaning and emotion.
Love subtext because it gives their visually entertaining movie more depth, meaning, and value.
Love subtext because they get to live the deeper experience of the characters, while experiencing the story beneath the story.
All of this means that PRODUCERS and AGENTS look for subtext when they read screenplays!