Bubbles are important in all aquariums (both freshwater and saltwater) that have fish inhabiting them that do not breathe from the surface. The bubbles raise the level of oxygen in the water, and without enough oxygen many fish will die.
In both fresh and salt water aquariums, bubbling and other gentle forms of turbulence, as well as live plants, allow the water to absorb more oxygen, which the fish need. Aquariums with more oxygen can sustain more fish.
Bubbles in a saltwater aquariums do several things, first it airrates the water raising the oxygen level. Next it carries protien through the aquarium which certain fish like to eat, such as the yellow tang. Lastly it keeps the water fresh by circulating it to the top where it gets filtered.