Conducting
If an orchestra is playing music, it is important that they all play exactly together.  They need to know exactly when to start, what tempo (speed) to go, how loud or quietly to play, and what the mood of the music should be.  If a small number of people play music together (like in a chamber orchestra), they can talk about this amongst themselves. One person can nod with his/her head or with the bow of a string instrument to help the group to start and finish together.