I have been told to put a metal skewer through the jacket potatoes before I bbq them by my moaning father in law...
Does this make any difference and if so whyyy!:)

Cooks more quick... heat goes through metal heats inside... I think the potatoes are fluffer when they get done.

Yes it does but you can just take a fork and stick the ends of the potatoes and it will accomplish the same thing. By putting a hole through the potato you also reduce the chances of the potato exploding. Honest. It happens in the oven too.

I have always done that when camping. They cook more evenly and take less time.

I had a gizmo once that had a handle... sat upright and you stuck the potatoes on each prong and put it in the oven... yum it worked well

I'm from Idaho where the Spuds are too dam big for all of that nonsense. We just start a huge bonfire, back the truck in and roll er off directly into the fire. We keep stoking the fire with pallets and firewood for two hours. Done! Feeds 16 dozen people.