Looks like the advantage of this source is how the stone is naturally fractured into a desirable shape without needing any further shaping. Just dig them up and they are ready to go. It would actually be kind of rare to find a source like this.
My father had a whetstone like one of those in the picture. He used it so much that it developed a “dip” in the middle. His pocket knives ended up looking like mini scythes or even toothpicks he sharpened them so much. Strange obsession.
Indeed handy.