This question raises the more important factor of the two key macro groupings of exercises. On the one side you have cardio vascular training, while on the other side is resistance training. Both forms of training have a role to play within any structured training plan.
When it comes to creating a structured training plan, you always start with the outcomes or goals that you as an athlete want to achieve.
Looking at a rowing machine specifically, targeted strength training, combined with a correct nutrition strategy, will boost upper body muscle gains and make you stronger rower. Using the rowing machine will boost muscle capillarity within your stronger, bigger muscles. Increased capillarity in turn helps with oxygen and glycogen delivery to the muscles and sets you up to make more gains from your strength training.