You honestly can't figure this out for yourself? It's not all that complicated. It allows JD to produce more of the same component (engine) lowering costs for design, tooling, manufacturing setup, prototyping, manufacturing production costs, inventory of replacement parts, etc. In manufacturing it's called economy of scale - producing greater numbers of the same thing lowers the overall cost.
This translates to more profit, or lower product cost, or both.
Think about this. You can go to a JD dealer and order parts for tractors that are 30+ years old. Do you have any idea of the amount of money JD has to dedicate to parts sitting in warehouses to provide parts support? It's probably in the hundreds of millions of dollars for world-wide parts support for agricultural, construction, and forestry equipment.
If you have the same motor in 3 or 4 different models, you're not carrying a whole lot of different parts in inventory for unique engines in every model.
It makes a lot of sense on multiple levels.