A few nights ago, I mistook a curb for an entry point to a parking lot. In so doing, I broke the plastic bottom engine cover on my VW Golf. I removed what was left of the part, and it seems to have not made a difference in the way the car runs.
Does it need to be replaced?
Yes. That under-engine pan not only smooths airflow on the highway but also protects under-hood components from excessive water spray when it rains and from road debris that can be kicked up from other vehicles on the highway. It can also keep snow, salt, and dirt away from the accessory drive belt, plus keep the alternator dry during rainstorms. In some cars, the under-engine pan helps with engine cooling by properly managing airflow.
Even though the under-engine pan on your car (and on many other vehicles) is made of plastic, it may also help protect fuel and brake lines. On some vehicles, this protection is provided by metal plates that cover other drivetrain components in addition to the engine.
If it were not important, vehicle manufacturers would not waste money on this part, which is generally out of sight and often must be removed before some maintenance procedures, such as oil changes.
This raises a final point. In some cases, a vehicle is returned after an oil change with the pan left off the car. It could be an error or a time-saving move on the part of the service provider. Either way, owners should check to make sure that this belly pan is still under the engine before leaving the service shop.