What was the role of the orchestra in an opera?

It is exactly the same in the case of the opera’s orchestra and its conductor with one solitary difference: you see this machinery and its director and can watch them in action. There is only one great theater in the world, the Festspielhaus in Bayreuth, where the orchestra and conductor are completely invisible to the audience, though this arrangement has been imitated by a few smaller houses, notably the Prinzregenten Theatre in Munich. Wagner was the first to realize this idea, but not the only one of his time to conceive it. Verdi, in his letters, earnestly recommended having an invisible orchestra...

Explanation:

variety in color and variety in texture