In some cases the actors aren't the only ones who are rehearsing. Again in musicals it is common to have a band of some kind to accompany the actors. Therefore it is necessary to have rehearsals for the musicians who are playing for the show. Their rehearsals are usually separate from the actors and they almost have no interaction until after tech week. These rehearsals are usually lead by a conductor. The goal is to learn the music and make sure that they are playing it correctly.