1.The power of apparently influencing the course of events by using mysterious or supernatural forces.
2.Used in magic or working by magic; having or apparently having supernatural powers.
3.Move, change, or create by or as if by magic.\
It all depends on what context.
The key signature allows the composer to use less accidentals which makes the music less cluttered. Just like the other person who answered this said when a score with multiple instruments is written it is almost always all in the same key to keep the music organized (their answer is better)
They can produce different types of theatre and make things more creative by the many different skill sets they have.
