Originally Posted by Alexus

Ditto on the aero effect, but what some people fail to remember is that the cover is commonly called a "splash guard." Not only will it improve the aerodynamics of the car, it will also prevent unwanted debri or pests from entering the engine bay (if conceived correctly).

I do have to agree, however, that Nissan went into overkill mode with the 16 bolts and 3 clips (which will probably be broken by most after the first oil change). It's a pain.