the black plastic is actually for the aurodynamics to deflect the wind going beneath the vehicle to keep it from going head into everything hanging below the body line at higher speeds, but if you have a plow out front I dont guess it really matters.
i checked that out and on mine there doesnt seem ayhting that it owuld bother and i dont have a plow but there is a big metal sheet uner there to hilp with that too. but if i notice adifference i'll put it back on it just looks alot cooler wihtout it and they take them off to put the plow frame on so they cant be that important
they were originally designed for the older trucks to push the wind below the lower front A frames, crossmembers and whatnot to make less of a wind drag. I have seen tests done on the that proved they did have enough difference in resistence to warrant putting them on the front bumper for wind deflection.
Yeah, if you look in the front of most cars and trucks these days you will see them in various sizes depending on the make and model of the vehicle, but yep they are there to cut down on the wind drag.