Aeration in aquariums happens mostly at the water's surface, and is aided by surface agitation. The little waves or ripples in the water's surface create friction with the air thereby promoting gas exchange and oxygenating the water.
The outflow of most filters creates sufficient surface agitation in most cases, and the addition of an airstone does little to increase oxygenation above and beyond what the filter already does -- and are therefore pretty much purely aesthetic.
An airstone will help if there is no other means of surface agitation, but again, if your filter already provides this, the airstone won't make much of a difference.
My fish do like to play in the bubbles sometimes though.
Best!