Why did they turn up the left side of the slouch hat?

The left side of the slouch hat is turned up to allow for the easy use of a rifle and to prevent it from obstructing the soldier's view when aiming. This practice is particularly associated with Australian and New Zealand soldiers, especially during World War I, as it became a distinctive feature of their military uniforms. Additionally, it serves a practical purpose in hot weather, allowing for better airflow and comfort.