Nitro cards are used as a seal between the powder and the shot. They can be used in place of a plastic shotcup in fiber wad loads.
Most modern load designs use nitro card (reduced diameter) wads within a shotcup for managing space. They can also be placed over the powder and underneath the wad to push up the entire shot column. We've even used these .070" as overshot cards. They produce better crimps and serve as a valuable spacer.
Nitro card wads are constructed of a specialized material selected to withstand the tortures of slow-burning powders and heavy payloads.