In an orchestra, is a conductor really necessary? What is the purpose and the responsibility of an orchestra conductor? What exactly does he do?

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yes becuase they have there ways

yes, they are sort of necessary, I'm in my school's orchestra. Usually the conductor cues you on when to start, or if your playing to quietly, stuff like that.

They are the brain/nervous system of the orchestra. Without them, musicians wouldn't know how to move the phrase, when to be quieter for effect, when to cut off to be in time, certain cues are give ONLY when the conductor says so, certain vibratoes are the conductor's idea.

They use hand motions to tell the players to play faster, slower, louder or softer. There's a thing in music called a fermata that means to hold a note until the conductor cuts them off. Also, if a section has quite a bit of rest, they cue them when they're supposed to continue playing.

They keep the rhythm and actually do teach the student/player, so yes I think they're necessary.