The function of the sabot was not only to impart spin for rifled guns, it also sealed some of the gap between the diameter of the projectile and the inside the canon barrel. This allow more of the force of the explosion of the powder to be imparted into the forward motion of the projectile, rather than to escape around the projectile.
The type of sabot depends in part on the type of ammunition. The sabot of the Hotchkiss and Schenkl projectiles were afixed to the middle of the projectile to give more stability. The Schenkl used a paper mâché sabot, which was rather effective, unless it got wet. The paper sabot was less dangerous to troops positioned between the canon and its target, as compared to having pieces of hot metal rain down. A good book is "Civil War Artillery at Gettysburg" by Philip M. Cole.