it's their job to do a road test where they take them out on public roads. They don't have a test track at the plant. Yes it's their job to do random testing on vehicles. from what i remember they look them over with a check list, then drive them listening for squeaks, rattles, and other various noises that might be found. while driving they're trying to also find problems that might be found only while driving a car. If they find a problem they try and make certain there's not a production problem. remember there's many people, parts, tools, automation that goes into building a vehicle so all these have variances. humans make mistakes, machines and tools in time wear and need maintenance. there's many inspection points to try and be certain a quality product is delivered.