It can play a crucial role -- some films much more than others.
It's especially important in a really quiet film like Sling Blade or Trust or Brokeback Mountain, where long interludes can pass without much dialogue, and much of the film is conveyed through pictures.
The score was just stunning and heartbreaking in Brokeback. But also soothing and joyful. And I noticed it was usually limited to the sequences where the two men were together during their two to three weeks out of the year, out in the gorgeous Wyoming back country, and blissfully together. It artfully conveyed the sweetness and quiet intimacy and joy of their life together.
That's a whole lot to convey. It was brilliant.