Most electrical engineers use “reference designs” published by the IC manufacturers to design their PCBs, although any open source commercially available design can act as a reference. They essentially copy the schematics from the PDF (or import if file formats/converters allow), ripping out whatever they don’t need, then reroute the PCB using their layers.

In some cases, when their PCB fab layer stack up is similar enough to the original board, they can go a step further and copy paste most of the PCB into their design so that any signal integrity work carries over. Realistically this is only really practical for low speed designs but still useful for a whole class of electronics.

I don’t use KiCad but software like Altium support modular schematic sheets and PCB rooms so theoretically it can imported into that (since KiCad’s format is open source S-expr)

I could imagine small companies that rely on these boards and that also have their fab and sourcing pipelines set up would be able to easier source these themselves. Just have to generate the Gerbers (fabrication output format most manus need) and then send it off as part of a larger order, etc.

Especially if you're able to replace certain small/passive components with those you already have in bulk, it could be a potential cost cutting measure.

Just a guess though.

For my case, they'd be useful if I wanted to know how certain subcircuits are designed or laid out.

Even for beginners, taking it into kicad, enabling the selection of only tracks and vias and deleting them all, then doing a full re-layout of the board as practice would be a cool project if you're wanting to learn.

I've got some projects that use ADCs on Pi "Hats" that connect to controls, I could see a future version which integrates that ADCs and pots directly onto the board to get a slimmer profile. It's quite handy, I wonder what the unit cost is with assembly at JLCPCB.

Yes to the first half, for the most part.