Swampy pretty much nailed it. :thumbsup:
With an elongated bullet a fast spin is needed to create a gyroscopic effect to keep the bullet from tumbling, or turning end over end and leaving a "keyhole" shape on whatever it hits.
With a roundball the spinning from the rifling is not needed to keep the ball from tumbling but it plays an important part in keeping the ball flying in a true direction.
The surface of a roundball is not smooth and uniformly shaped although it does look pretty good to the human eye.
To the wind at subsonic and supersonic speeds it looks much different. The uneven surface will develop different pressures from the air blast and if the ball is not spinning (like it was fired from a smoothbored gun) these unequal pressures will cause the ball to be pushed off course. The direction it will fly is impossible to figure out.
If the ball is spinning about the axis of its flight these same pressures are trying to deflect the ball however because it is spinning the actual direction of deflection is always changing.
Put another way, lets say that at one position the pressure is pushing the ball to the right.
If the ball was not spinning it would start going to the right immediately after leaving the muzzle.
With it spinning, just about the time the pressure was going to push the ball to the right, it is now trying to push it downward to the right, then down, then down to the left, then to the left, then up to the left, then up, and so forth.
The deflecting pressure is always changing its direction about the axis of the balls spin and along the axis of its flight. The net effect of all of this is the ball is not really deflected at all so it continues to travel along its trajectory towards the target.
To accomplish this great task the ball does not need a super fast spin so the slower rate of twist works fine.
The slower rate of twist does another good thing for a patched ball. It does not tear up the soft cloth patch even if some very high velocities are used.
This makes a slow twist barrel much less sensitive to high or low powder loads (velocities).