My key philosophy on writing, and art in general, is it should always be used to say one thing. One idea. One important lesson you want to teach someone.
Having learned a lot about life through the filter of music, I can’t help but equate writing to the relationship between melody, harmony and rhythm: The main idea of your story - your theme - should be like a strong melody. It’s what you want stuck in the audience's head. This theme can be twisted, inverted, reversed, contrasted etc... but everything you do in a work of art is meant to deliver this one idea, this melody, to the audience.
Harmony and Rhythm are important in the fact that they work to support your melody. In writing, these are things like emotions, steaks, adding time constraints for tension, using descriptive details etc... A nice harmony is good in and of itself but it's not a strong piece of music. A good rhythm is nice too, but again, in every popular song, it’s only there for support.
Through my adventures, I’ve found that this idea of simplicity in a message is in every form of successful art. In photography, the main goal is to get the viewer to look at one thing. Subjects are highlighted by way of using framing, the rule of thirds, contrasting colors, layering the image etc... all of this just to make the subject pop. In comedy, it's leading the audience to listen to your one punchline at the end of the joke. In business: all of the advertisements, sales promises, value stacking, branding etc... is all just to say, "I can do this for you."
Thanks for your time,