Diesels have a fairly large undertray, most petrol cars have a shorter one except for the LCR which has a much bigger one with NACA ducts in it.
It affects aerodynamics, both in smoothing out the underbody airflow for better drag reduction and routing the cooling air through the engine compartment. The ducts on the LCR tray direct air to various important bits.
It does play a part in noise reduction, being lined with sound absorbant material. Which soaks up oil a treat.