This approach may be used by manual control units that have an integrated ground control station (like the UAVComponents MicroNav shown below). Joysticks are also commonly used to allow developers to fly the vehicle in simulation.
Radio Setup is not required if using a joystick (because a joystick is not an RC controller)!
QGroundControl uses the cross-platform SDL2 library to convert joystick movements to MAVLink MANUAL_CONTROL messages, which are then sent to PX4 over the telemetry channel. In consequence, a joystick-based controller system requires a reliable high bandwidth telemetry channel to ensure that the vehicle is responsive to joystick movements.
Information about how to set up a joystick is covered in: QGroundControl > Joystick Setup.