>>1260952When it's windy it can make it more ærodynamic and less likely to blow off, also in thick bush or overhanging branches it streamlines the hat so it's less likely to get snagged, and finally if you're doing manual labour (particularly if you're shouldering a load) it keeps the brim out of the way. Aussie soldiers wear a slightly classier "slouch hat", a type of bush hat that they typically button one side so they can shoulder their rifles.
