I don't think you should have to book shows on any of the other ships. However on Iona, so I guess Arvia too they let the entertainment staff know how popular they are. If everyone hasn't been able to book the first night they will repeat the show. This is possible as the other acts can perform elsewhere.
For the restaurants it is much easier to book rather than traipse around the ship only to find where you wanted is full and have to set off somewhere else.