When they use music to create mood, they will use it to create tention and different emotions like, sad moments, scary moments, intense moments, etc... So when he used music to create a intense moment or a sad moment he is wanting the view to feel that emotion by adding in that music, but let's say he didn't add in the music, when ever he doesn't add in music we can't think of what type of tention he had in mind, becouse certain sounds can be used for certain stuff.
Hope this helped
