Bubbling 'air' through your water will tend to make the Co2 and Oxygen levels match those in the air around the tank. If you have oxygen rich water then bubbling air through it will lower the oxygen levels, if you have Co2 rich water (such as someone who injects Co2 might), then bubbling air through it will reduce the Co2 levels in the water.
If you have surface disturbance from your filter then the fish have all the oxygen they need, and the air curtain is purely asthetic. It will however also add some additional flow and movement to the tank, which is why I use them.