The higher the salinity the higher concentrations of Cal, Alk, and Mg you are adding with water changes. It can help a little, especially with trace minerals and when you have quite a bit growing. It isn't a huge difference, but going from 1.025 to 1.027-28 can be a 10% difference. If your parameters were 10% higher and water changes helped pull them back up to that point, would that make a difference for your corals? It you already dose then you might not notice a difference, but if your parameters are a little low it could make a big difference. Many people do not dose trace minerals so that little boost could help if you need it.