Glad you asked. It involves less ductwork, simpler duct runs, easier system expansion, less demolition if switching from constant volume to VAV, a maneuver for handling solar load shift, and more. For when to use it, why to use it, and how to design it, read on.
ANSI/ASHRAE Standard 52.2-1999 provides a new selection tool. A member of the authoring committee explains the Standard and introduces MERV to the process.