I’ve never entirely understood the point of the Appalachian League, or its counterpart, the Pioneer League. They’re classified as advanced rookie ball, a level at which only 18 teams have affiliates, compared to 30 at all other levels. That’s somewhere in between the New York-Penn League, which is intended mainly for college draftees, and the Gulf Coast League, which is intended mainly for prep draftees and players moving up from Latin America.
The Pirates have utilized their affiliate at Bristol for developing prep pitchers. Of course, that’s typically just 2-3 players per season, or less than a tenth of the roster.