The mudflaps on my Rogue did nothing to stop the meth head from plowing into it, greatly reducing longevity. You can also see they didn’t stop a mist from dirtying the car, but did cut down on buildup that would get your pants dirty.
I have them on our Chrysler Town and Country to keep the pants of the occupants clean. The front ones make a difference and the rear ones balance out the look.
My pickups have always had mudflaps on them, and they all regularly saw gravel roads. I believe they helped keep rock chips down.