s in the US Air Force
(and presumably those of other countries) use tactical call signs
to communicate with their fellow pilots while airborne.
During Vietnam, it was discovered that in the heat of combat pilots would forget their official flight call signs, which up till then had been assigned randomly on a daily basis for security reasons. So when a Mig was sighted, someone might call "Buick break right," meaning the guy who was "Buick" yesterday. Today's Buick would then break right, the guy you wanted to break right would stay where he was, and the wrong people (us) would get shot down as a result.
Since you always knew who your wingman was, pilots started using first names to prevent this type of confusion. If you wanted Bill to break right, you'd call "Bill break right" -- which worked fine, until you ran into a situation where you had two or three "Bill"s in the sky around you, all breaking right.
So the practice of using individual nicknames came about, and the problem was solved. Since then, tactical call signs have taken the form of names such as Hawk, Truck, Saint, Obi-Wan, Rooster, and so on.