

That part is responsible for 3 things:
- Noise. It reduces the amount of turbulence under the car
- fuel efficiency, same as Noise
- protection from the environment. It reduces the amount of dirt and water that hits parts like the engine and other stuff there
It is not critical and as others said, you could leave it of. I would recommend to have a look of there was any other damage done while they ripped off, as the weather allows to
First of all, this would be illegal in many countries.
Second of all: we can differentiate cars by: has red lights, back.
If we lose this option we can no longer differentiate easily if there is a car coming towards us or driving away from us.