1. Lowers manufacturing costs. If there were a Law of Manufacturing it would be this: the less differentiation among the widgets, the more cheaply the widgets can be made. That thinking was behind Ford saying you could have any color you want, so long as it's black. Note this doesn't just affect assembly, it affects the supply chain as well. Your supplier only has to provide one kind of seat. The company making the heating units can crank more out and reduce the unit cost of each.

2. Increases supply - which reduces cost. As I said above, heated seats can be sold as an option. Any owner of the car can buy the option at any time. That increases the supply of cars in the used market. If I want a car with heated seats I can buy the model where that feature hasn't been enabled and enable it myself.

You mention you can't pick and pull the hardware. There's nothing in principle preventing that. All the hardware is the same. If for some reason your seat were to become damaged, you don't have to look for a heated seat. Any seat will do. If BMW is doing anything to prevent that then they're going to run afoul of Right to Repair laws.