But your equipment for handling water is almost certainly going to measure things in terms of volume/minute, where the volume unit will not be acre-feet. Perhaps it's gallons. Or hogsheads.

The acre-foot is a useful measure only for certain scales of volumes, but there's a need to convert between small and large scales of volumes all the time.

If we didn't have the metric system, sure, the acre-foot makes sense to use. And in the metric system, perhaps people will occasionally talk in terms of hectare-cm instead of liters or m^3 (I don't know enough about non-US water systems to know what they typically use). But regardless of liters or m^3 or hectare-cm, these can all be converted by moving a decimal point rather than multiplication or division with multiple significant figures.

We have a better system, we have just chosen not to use it.