A showboat must have been a pretty lucrative business back in the day. I think a lot of us forget, but traveling wasn't always as fast as it is now. I imagine that a theater troupe traveling the regular way probably couldn't go very quickly. Then they would have to construct their set in every town they stopped in.
A showboat solves both of these problems. The set is already constructed on the boat, and travel is taken care of. Whoever thought this idea up was a genius!