What is the purpose of RobotMap.java in CommandBased?

[techplex]

Q: What is the purpose of RobotMap.java in CommandBased?

I understand that the actuators, motor controllers, and sensors are all instantiated within. But I feel like it would be more logical to instantiate them in their respective subsystems?

I have tried instantiating the motor actuators etc, in the subsystem and the code does work as expected.

I guess what I am really wondering is what principle of Object Oriented Design, (or any other reason) does RobotBuilder scaffold the code in this way? Does it make the code more maintainable somehow?