Vinyl Siding over Metal Skin

I’m about to re-skin a number of my POH’s. The material supplier suggested putting fan fold between the vinyl siding and the mobile home metal skin. I understand the fan fold has some insulation value and may provide some water barrier. But, given the fact that the mobile home is already insulated and water proof, is there really a need to incur that extra cost. I would appreciate comments from anyone who has already gone through this. Thank you in advance.