Acting in some theatrical productions can be a valuable experience for any actor. Many actors consider theatrical acting the purest form of the art.
Unlike movies or TV, if something happens during the play (a falling set, forgetting lines), the actors are forced to stay in character and improvise as best they can. These added pressures help actors grow and develop their craft.
Plus, because theater actors play the same role day after day, they can experiment with and fine-tune their character development. An actor is truly in control of his or her performance in the theater. While in television or film, the director has the final say of which cut will be used.