These sort of hull vents were common when leisure boats had air cooled Lister SR2/3 engines under a cruiser stern deck. Often there were openings on both sides of the hull, one as an intake (for combustion and cooling air), with the heated cooling air being ducted out the opposite side.
I suspect that for some boatbuilders they just became standard practice, although with the change to water cooled engines nowhere near as much ventilation is needed, and what is still needed can probably be adequately supplied by the gaps around deck panels in most cases.
That sounds like a lot. But since it all gets exhausted through a 1.5" or 2" pipe, it is clear the combustion air intake doesn't need to be very big.