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I've noticed today's trucks have this 3-4" plastic guard that runs along the bottom of the front bumper. It strikes me as something that would hinder ground clearance or otherwise brush into things.
What purpose do these serve? Why have they gotten so much bigger recently?
You can see how pronounced it is in the following trucks:
While on these older trucks it is less noticeable, or seemingly even a natural extension of the bumper.
As @MooseLucifer said in the comment, these deflector strips are generally there for air deflection. They do two things - one, they change the air pressure under the car helping stability at speed, and two, they (sometimes) help divert airflow up to the radiator.
In my experience, you always end up leaving it somewhere in the woods by accident.
The more air allowed under a vehicle the greater affect this air will have on fuel economy. The lowered front air deflector will reduce the amount of air and thereby improve fuel economy.
The large over the road trucks have several new air types added in recent years for the same reason. The most noticeable has been the long skirts under the trailers.
It is an appearance extra so the dealer can sell you something ( for pickups) . I added one to my Nissan Titan ; It was an extra offered on the QX 56 suv, so it just bolted to the Titan ( at the time it was not offered on the Titan).
