I'm not an automotive or fluid dynamics engineer, but very likely aerodynamics is at least part of the reason the factory installs them. The way air flows under a car definitely has impacts on drag. See: https://gr8autotech.wordpress.com/2013/06/16/vehicular-aerodynamics/ Jeeps are about as aerodynamic as a brick, so they can use all the help they can get. Manufacturers are pressured to meet legislated fuel economy numbers, so they'll spend a few bucks in plastic to shave a little MPG off. Probably not something you can test at home with a Jeep (too many variables) but a wind tunnel test would show the difference.
Just speculating here, but it might also be for thermal management. Front air dams help air reach the front brakes, and they influence underhood temperatures. For example, I had a VW Fox, which is a bit of an odd duck because it's a FWD with a longitudinal engine (same orientation as a Jeep). Well because of that, the oil filler is nestled right up by the grille -- and if you take the cap off a Fox you may be alarmed at all the creamy white oil-water emulsion you'll find stuck to the underside. It doesn't mean you have a failed head gasket mixing oil and coolant, it's just that the oil there never gets hot enough to burn off any moisture. Underhood airflow is something automotive engineers do, or at least should, take into account.
Finally, plastic in that area on most cars is designed to make an awful noise if you start sliding up onto a curb or something. So at least at slow speeds, the driver is alerted to stop before damaging the oil pan or other important bits.
All this to say, I don't think deleting the plastic from a Jeep makes any practical difference though.
It pushes incoming air flow under the axles reducing drag, probably helping with cooling at low speeds by creating a low pressure zone under radiator area helping to drag hot air away.
It is helpful off-road driving through high grass or soft snow forward, but becomes a problem in reverse, it can be quickly removed though.
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