That's because it's the quickest practical way to achieve the scientifically-proven desired setting for a conforming engine, cleared.
PS: The single most important reason for my decision to attend an APS course was the opportunity to meet and draw on the wisdom of a person who's forgotten more about piston aero engine operation than anyone has ever learned in the third world of GA that is Australia - Mr John Deakin. You can't do that by reading on-line materials.