I think that swimmer jockstraps were and still are intended for wear underneath swimming shorts that do not have a liner or that have a liner that is not adequately supportive.
You don't want the wide waistband of a regular jockstrap sticking up above your shorts when shirtless and you don't want all of the water-trapping fabric of cotton briefs.
The swim jock is the only kind of jockstrap I see sold in some stores anymore.
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