Boonie is a slang term for wilderness, think "out in the boondocks." These have always been popular headgear choices for military personnel and even have their origins with the military. This slang term from a military point of view means "Out in the Field" which is going to mean that you are not on base anymore. During the Vietnam War, this style hat became regularly used by US soldiers. Although they were popular amongst the soldiers, they weren't issued to them by the military, instead, these soldiers would buy the hats from local tailors. Today in Vietnam, these hats are very popular still and commonly made by a local tailor with various brims and fabric patterns. While this country mostly sells them as a popular tourist gift, they remain highly functional.
The soldiers of The Republic of Vietnam would wear these hats and the US Special Ops took a liking to them. The lighter weight and less bulk make them much more comfortable over the metal helmets as they offered better airflow and tactically helped one hear much clearer than how sound would flow with the metal helmets. While these also offered superior camouflage properties they were not the best for protective purposes and so when on guard duty or in a foxhole the helmets still came in handy and were highly useful. Because this hat originated in a jungle atmosphere the hat can also commonly go by the name "jungle hat."
With time, the US military saw the benefit of these hats and took them in as their own standardizing them. They became widely distributed and used by the US Military in such high conflict areas as Iraq and Afghanistan. They are so popular with the US Military that each branch of the service uses their own unique patterns. Aftermarket vendors who have obtained these hats are common sources to obtain military standardized camo pattern hats for public consumption and they are similarly very popular with the public as well.
Outside of outdoors activities, these hats are worn by professionals including Military and Law Enforcement SWAT teams. They offer excellent practicality and they can make outdoors environments much more comfortable and pleasant. They are easily packed and carried or worn around the neck and they tend to be durable enough to be used hard.