Harmonizing Melodic Key Motifs
If you have a melody for example in the trumpets (it works in any melodic writing but it is most prominent on brass themes), it creates a very nice colour to harmonize the melody on key motifs or phrases.
Coming from all trumpets playing the theme together in unison to splitting them up into a chord for a few notes creates a fantastic new sound colour. Once again, the Star Wars Main Theme is a great example for that. While all trumpets enter the theme in unison after the intro bars they split up for the last 4 notes of the theme at around 0:16, which is a perfect example of the above described concept.