Beginning as most things do, out of necessity, beanie hats were first worn by blue colored workers such as welders, mechanics and sailors to keep their heads warm and their hair out of their eyes. From there it made sense that anyone in cold temperatures, with or without hair, could wear one for the benefit of their own body temperature.
Also known as a knitted cap, skullcap, toque or toboggan, the hat has become as synonymous with fashion as it is with function. Just don't be one of those people who wear them all summer long. That's just not right.