Military boots are designed to withstand the demands of actual combat or combat training. There are designed to provide maximum traction, ankle stability, and protection for the foot. Tradionally they are made of hardened and waterproofed leather.
Today military boots are made from technologies originating in hiking boots, like Gore-Tex nylon side panels, which help with ventilation and comfort. They are also specialized for different climates and weather conditions, like jungle boots, and cold weather boots.
Military boots and shoes have become very popular in secular society as well. Especially with goth, punk, metal, and industrial subcultures. Other choose to wear them simply because they are so tough, durable, and comfortable.