What sort of tank and filter do you have? What sort of fish? Do you have plants?
From what I've read, airstones help oxygen exchange by disturbing the surface of the water. If you've got a filter that dumps the water back into the tank and disturbs the surface as it does so, then you probably don't need the airstone to add to the surface disturbance.
Some fish like currents and like playing in the bubbles. Some fish like still water. If you've got fish that like currents, they might like the bubbles.
Some say that you shouldn't run an airstone during the day if you've got plants because it depletes CO2.
After reading up on it, my conclusion was that I probably didn't need it. I've got one in one of my tanks because it looked stagnant. It was basically an aesthetic choice. I haven't got any in my other big tank, or in any of my betta tanks and things seem ok so I don't think they're missing out on anything.