They provide a quick and easy way to cut monofilament line, a common entanglement hazard for divers.
Men are stupid about knives.
It's one of those subjects like guns and fast cars that gets the old testosterone flowing. And we all know what effect that has on clear thinking. As a result, too many dive knives look like Steven Segal action figure weapons. They come finished in black, too, for midnight murder. Most models are also available in sharp-pointed versions.