Aside from the machines, coal seams vary widely in quality, depth (moving upwards and down and laterally) and size (thick seams becoming thin, and vice versa). In order to prevent the machines from digging “rock and dirt” and spending extra time and money in that way degrading the product that they’re trying to extract and sell humans are needed to direct the machines: “dig this way, not that”.
In underground mines, too, the quality of the entire strata has to be watched to determine when and where to erect shoring, pump out accumulations of water, add lighting and tracks for the ore vehicles (and people movers) ... or move all of that from an abandoned shaft to a new one.