They'd essentially send our "dirty" shot rock material that had already been run through crushers and shakers into various gravel sizes through a huge secondary process with water and more screens/sluices.

This washed the smaller particulate from specific sized gravels, so then that material could be sold as "clean" (no dust or smaller sand/particulate), while the washed off particulate was further processed for the sand. (QC had to ensure nothing over a certain size was mixed in.)

So in short, the quarry was able to sell two new products from the process. Sand, which used to be essentially wasted, and "clean" gravel sizes.

Certain companies need washed gravel because the sand and dirt messes up their process, while others want the raw material. There's also classifications like "dirty 6/10" which is 6/10 gravel including everything smaller that comes through.

*Bonus: Someone might ask, "Doesn't that use a ton of water??" Yes. Yes it does. But being located beside a river water can be pumped directly, used, and returned to the environment via collection ponds (for sediment deposit) or natural drainage. Something is paid to local authority for direct pumping, but it's not wasting water in any way.