Ditto Daryl's comment on keeping the roundball centered in the bore.

This is how I prepare my wadding for one of my bore rifles.
Note the ball in the tweezers with the felt wadding.
It's sitting on a ring-shaped wad which is sitting on a disc-shaped wad. The ring-shaped (or Donut shaped if you prefer..!) wad expands against the barrel wall under pressure, sealing the wad column and keeps the ball centered on it's way into the throat.

Also note the red card-wad for separating the powder from the lubricated felt wads.
I form them in the steel ring from discs.
These cup-shaped card wads go in the cartridge with the 'cup' shape facing the powder.
Pressure 'inflates' the cup-shaped card, further contributing to consistent gas-seal.

Here's a cut-away of a WR Super Explora (magnum paradox) cartridge.

And here's a diagram from a mid-19th century text showing typical bore-rifle cartridge load column.
Powder against bullet!

The two 'rifle' loads are just shown for academic purpose.
Both show flat-based bullets.
The roundball needs something to account for the hemispherical shape of the rear end of the ball -- simple flat wadding will 'wrap' around the ball and compromise both gas-sealing and bore-sealing properties of the lubed wad column.

Cheers
Tinker