Not an expert, but what you describe is testing different parts of the system. Real world data tests that the sensor system is creating an accurate model of the environment, whereas simulated data ensures that the vehicle makes correct decisions for any given input model.
e.g. real world data tests that the vehicle can detect a stop sign. Simulated data tests that the vehicle stops at a stop sign.
e.g. real world data tests that the vehicle can detect a stop sign. Simulated data tests that the vehicle stops at a stop sign.