the typical case is where you have hundreds of pppoe user on your local network and 1 connection to your ISP (public interface).
So you are trying to shape traffic of all pppoe interfaces (virtual interfaces on your local physical interface), there you need the global-in/out.
As the shaping on a physical interface just make sense on the outbounding traffic, I would like to know how to use the global-out and GLOBAL-IN to shape correctly such a situation.