A nato strap is a one-piece nylon strap that slips over the spring bar under the caseback and back over the other spring bar. Its purpose is to provide redundancy; if one of the spring bars were to fail (break), the watch wouldn’t fall off the wrist and remaining hanging on your wrist by the one remaining spring bar.
You may be wondering what good is a wristwatch that is left flapping around on your wrist? Consider the usefulness of this under diving conditions. If your watch gets snagged underwater, causing a spring bar to break. The watch would slip off your wrist and go dive bombing right to the bottom of the sea before you could notice it.
The nato strap’s other qualities are comfortable for extended use, waterproof, lightweight, nonreflective, inexpensive, suitable for hot weather, making them ideal for military use.
Modern nato watch straps have moved away from their functional cosmetics. They come in a dazzling array of colours, country flag natos, bond natos, plain natos, and NATOs in premium ‘HD’ fabrics. HD because they are natos woven in ultra-thin threads to give them a seatbelt like texture.