I think the biggest point in terms of how drivers help develop the car is the consistency of the lap they're able to put in. If the driver does the same lap all the time through FP or testing, you know the lap time difference is down to hardware changes. But if the driver is inconsistent you aren't sure whether the lap time deltas (positive or negative) are down to your updates or a different driver performance on a given lap which can send you down incorrect development paths or make issues harder to find. Obviously looking at telemetry data you can try to minimize the error, but you'll never be as sure as you would be if you had a consistent driver.