It contains ionic minerals and trace elements naturally found in seawater, that are constantly consumed by corals, fish, and other aquatic organisms. These elements are needed to be replenished on a regular basis in order to keep living organisms in aquariums healthy and to keep their colors bright. (edited)
It provides the required growth of coral skeletons, oyster shells and other similar skeletal organisms.
It helps increase the alkalinity in aquariums, helping to restore pH stability