Your question is very appropriate. It would be very easy to order the plate to do it with and rough cut the blanks with a band say
The issue can be cost. Having a furnace and the ability to cast "Blanks" or Rods can be a cost saver.
A good example is these valve push rod valve gland.
I went out looking for a piece of bar stock to machine them from and found prices in the 60.00 per foot range for brass or bronze.
I ended up using my waste oil furnace to cast a billet from 2.00 per pound scraps and then machined the glands from that.
Total time on the pattern was an evening in the shop.
Casting time was two hours, Cost to run a waste oil furnace was Zero.
Having a furnace capability in your shop can open a whole new world for a modeler. It will definitely make you think out of the box.
This is a charcoal furnace using an electric leaf blower for air. I understand it is over 40 years old. I have seen it melt 15 pounds of Brass/bronze in 30 minutes for the cost of a big bag of charcoal.
It produced this in an afternoon.
The patterns for gear blanks can be done in an hour and the casting in an afternoon.
Old Pistons, engine blocks or transmission cases can be found in just about any junk yard in the world.
It will be interesting to see how he tackles this beast.
Keep the photos flowing on it.