It indicates scene changes because transitioning play scene changes to film scene changes can be difficult. For example, think about the music when the boat first washes ashore and you transition between Viola and Sebastian.

Music definitely sets the mood. Think about the difference between the dancing scene in Olivia’s house that takes place in the middle of the night, and the tune that Feste sings over and over again.

Highlights themes-definitely. Think about the music that swells when Viola and Sebastian are in the water, or the melancholy song that Feste sings.

If you aren’t talking about the movie, this question doesn’t really make any sense, because different productions may use different musical scores. Shakespeare didn’t score the play…=)

Good luck!