This piece sums up just a few of the arguments for sabots pretty well.

"Why use a sabot slug? There are several good reasons to give a sabot slug a try in your gun. First, the plastic sabot reduces the weight of the slug, thereby allowing the slug to fly farther and faster. This also means that for any given distance the trajectory is much flatter, allowing for greater accuracy (assuming all else is equal). Second, the sabot helps keep the barrel from becoming leaded and dirty".
NORTH AMERICAN WHITETAIL