Generally speaking Wildeboer, cardboard wads are used under the base of conical bullets as a gas seal so as to increase accuracy and prevent leading by protecting the bullet base from flame cutting/melting.

Also generally speaking, a wool wad is preferred over a card wad if one were to use a wad under a patched ball. A patch/ball combination that fits the bore well and shows no evidence of patch failure after firing is not likely to benefit by the addition of a wad. However, even with a well fitting patch/ball combination, a wad (either wool or card) may be desirable for a hunting load that remains in the bore for and extended period so as to provide a barrier between the patch lube and powder charge.