A conductor is to an orchestra what a pianist is to a piano. He is the one translating the intentions of the composer into instructions for the musicians. Classical music is not single-tempo, single dynamic. Tempo can change every bar and the possible ways to attack each note varies greatly. It would not be possible for a large ensemble to agree on how to play.
The main role of the conductor happens during preparing and rehearsals. The performance itself is the rendering of 100s (when not 1000s) of tiny decisions from the conductor (who can, of course, listen to suggestions from the musicians).
