Aviator sunglasses started out as an eyewear style popular amongst pilots. Then motorcycle police got jealous and bought them for themselves, and they became just as well known as cop sunglasses. Then a lot of the rest of us decided they were awesome and started wearing them, too. Well, they look super cool, are lightweight, and have that classic, authoritative look.
They were actually probably popularized most by actors playing pilots. Go figure. Grab a pair of aviators. Complete your steez.