A smooth crown allows loading sung combinations without cutting the patch.
Snug combinations shoot cleanly and do not allow fouling to build up.
When you load the next one, you clean the last one. If the loading gets tougher and tougher,
your ball and patch combination are not snug enough - or, you are not using a good enough
lube - or, not enough lube.
Because you do not have grooves (rifling), the ball needs to be smaller, to allow a thick enough
patch to be used. A thicker patch, holds more lube, making loading easier.