Frankly I could point it right back at you as the example of a good thing to have. If you need to dive underwater without equipment or cross smoke during a fire, it's useful to have a reference of how long you can keep at it, how many rounds does that take, how much distance you can cross, what happens once you can't keep at it anymore. We are talking about adventurers, it's surprising that this is somehow thought of as an irrelevant edge case.

Are we expecting that the player should always have spells or some magic scuba for this?

I really don't get what's with OSR and not wanting to roll. I'm playing an RPG, I'm up for rolling. Though in this case, the rule does not even require rolling until you are already drowning.

For the few times your players want to swim a lot underwater OR if you use monsters designed to drown them long term

Okay, explain to me why do you need rules for holding your breath in 5e.

Because water is generally everywhere and you might go in it? Surviving poisonous gases? Strangulation? If you wanted to point at rarely used rules there's a plethora of better options to pick. This is more like asking why do you need rules for combat.