Welcome to AA, biohazzrd!
Airstones help break up the surface of the water, which facilitates gas exchange - O2
in and CO2
out. Splashing from a HOB
filter or surface agitation from a spray bar accomplish the same thing too.
It's just that some people like the effect of a wall of tiny bubbles. Some fish like to play in the bubbles, but most really don't seem to notice them.