Sound can be one of the most challenging aspects of filmmaking, especially for indie productions. There's a reason studios build expensive sound stages; shooting on location is noisy. Throw in a mobile generator for powering your set, and you've got a 60dB engine (on the quieter end) to keep out of your dialogue, and those are just a few issues that come into play during production.
In post you face the challenge of making the most of what you've got, cleaning up the dialogue where you can, and using ADR (Automated Dialogue Replacement) where you can't. Despite all this, putting sound to picture shouldn't be written off as a task of diminishing return, it's half of your story. Or if you ask Danny Boyle, up to 80% of it. That sounds like a lot, and it is, but after considering some of the things sound can do for your film, you may come to agree.
In the most general terms, sound in film is responsible for two things: Advancing the plot by means of dialogue, and immersion in the world of the film. Starting with dialogue, it's pretty clear that no other part of a film does as much to advance the plot, and dialogue that is difficult to understand is one of the primary technical complaints a film receives (even over shaky cam). Within the sound industry it is a common mantra that the work of a sound designer is most successful when it has gone unnoticed. That's important for dialogue, and it also speaks to the nature of immersion. The audience should hear the dialogue naturally and clearly enough that they don't think about the way dialogue sounds, they just hear it and follow the performance. Similarly, ambient sound is often so subtle that you may never notice it in a film, but it exists to draw you into the world without distracting you from the plot.
The subtle power of sound is important, because it has the ability to influence you without your knowledge. With this power, sound plays a role in guiding the audience, particularly, by their emotions. One of the most easily recognizable forms of emotional sound is music. It can evoke almost any emotion, even without lyrics. Sound design does this too, by playing with associations that people bring to the theater. Running water is often capable of invoking a sense of calm, which can be emulated by the characteristic of white noise. Like music, sounds can harmonize pleasantly or cause discomfort via shrillness and dissonance. Cacophony can convey the sense of scale and chaos that you might experience on a battlefield, even if you have too few extras to make it look as big as it sounds.