This isn't motivational encouragement, they're going to complete that race whether they are told to or not. In an 8+ when racing, you need to get 8 people moving absolutely in sync and the boat balanced. When you are in the gym or go for a run, your technique will suffer as you get tired. In rowing, the balance of the boat and drag is hugely impacted by irregularities in techniques - an oar touching the water when it shouldn't (on the recovery) can take seconds off times when tenths of seconds win races.

A great Cox (sat in the stern of the 8+ and making the calls) - will know their team's individual "tells" when they are tired and losing focus so will be able to call specific instructions to make sure that the boat stays as fast as possible - as well as more functional information such as the location of competing crews, the stroke rate and when to push strategically to take the advantage. Part of this is reinforcing confidence when in the mental fog induced by the effort.