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Its function is actually quite simple: before a diver's descent, the 12 o'clock bezel marker is aligned with the minute hand, allowing the elapsed time, up to 60 minutes, to be read on the bezel (which is why quite a few dive watches come with a particularly prominent minute hand).
Originally, diver watches were designed to fulfil a purpose. It was to remind divers when they had to get back up to the water surface in a time when no diver computers existed. Ironically, the fact that divers' watches weren't considered particularly collectible in the past, makes them all the more collectible today.