More for the function of covering more of the tire that is still kiing out of the side of the Jeep lol.
I have ready that the reason most go with plastic is because the plastic will give and break if pushed hard enough whereas metal type flares don't give....which is good, but what is happening with the pressure that would normally bend the flare? Is it causing more harm elsewhere?
All in all it seems to come down to personal preference as with anything.