Using wood under the Aluminum

Hi All. I am about 2/3rds done with my Generic Benroy build. I have just finished insulating and have decided to put .040 mill finish aluminum on the sides and top.
My question to all is "should I use 1/8 Luan or Baltic Birch under my Aluminum skin on the roof / hatch. Why do the plans call out for using 1 pc of Birch or 2 skins of Luan of the same size?
My question to all is "should I use 1/8 Luan or Baltic Birch under my Aluminum skin on the roof / hatch. Why do the plans call out for using 1 pc of Birch or 2 skins of Luan of the same size?