Why is always that vital? If a unidirectional bezel is knocked and accidentally moved on the dive, it may only display a longer underwater time. Meaning the wearer will overestimate rather than underestimate immersion time, and in situations that need decompression stops, this can be a make-it-or-crack-it aspect that helps https://www.watchesworld.com/frozen-in-time-luxury-watches-for-winter/