You do actually need this on a Ford cab 13. it's the splash plate which goes on the step on the driver's side, just in front of the battery.
Yours has a slight fold in it which it shouldn't have. The cutout in it fits around the bit where the cast bracket on the chassis is bolted to the ridiculously slight step brackets.
I have a couple already so someone else (closer to you) may benefit from it if you don't want it.