I can't tell you exactly why round balls are patched. Most say its to get a good gas seal but if the ball fits tight enough it really doesn't need the patch. Ease of loading with a smaller ball is my guess. The patch grips the rifling and spins the ball. Some conicals are paper patched. Minies are never patched. They have a hollow base to expand into the rifling.
The patch cleans the bore each time you load. It grabs the rifling and grips the ball. Makes a better gas seal.
The mini is under sized, has a hollow base and loads easily even in a dirty bore. When it is fired it expands to seal the bore. Minis must be of pure lead to assure expansion .
I have never used conicals in a rifle. just round balls with patch.
Runes is right--use pure lead for conicals. I've use conicals almost exclusively--cast my own. Lead type makes a big difference--too hard and they fly all over the place, pure lead and they can be tack drivers.