Saltwater corrodes firefighting equipment and may harm ecosystems, especially those like the chaparral shrublands around Los Angeles that aren’t normally exposed to seawater. Gardeners know that small amounts of salt – added, say, as fertilizer – does not harm plants, but excessive salts can stress and kill plants.
It’s also very common in agriculture, especially older areas that use flood irrigation, where A. Larger volumes are hard to use in any other unit, and B. you want to know water application rates per-acre on your crops, something that is very easy to find when you are applying acre feet of water over X acres.
Yes it’s a “stupid” unit but it has it’s place.