When you add a DriveTrain and Controller device to the Robot Configuration Wizard, VEXcode automatically generates the code for how the Controller will command the DriveTrain motors. That code is automatically added to robot-config.cpp to make it easy for students to get started.
You’ll still need to write your own code for other motors, like Lift or Claw motors, to the usercontrol() function in main.cpp. They just made it an easy entry point for teams to get the Drive Control going through the Robot Configuration wizard.
They can take control of everything by enabling ‘Expert Robot Configuration’. @tfriez explains how to do that in this post: VEXcode V5 Text 1.0.1 - You Spoke, We Listened