THE GMT-Master was originally designed for pilots, and in that profession, setting the "home" time as GMT was useful. This is probably why the 24-hour hand is popularly known as the GMT hand. The rotating bezel of the GMT-Master (or Omega Seamaster GMT) allow another timezone to be set as well.
For the international traveller, it is more useful to set the "home" time to that of where you reside, and the current time displayed by the regular hour hand.
Of course, if you don't travel, you can set the GMT hand to a timezone that you may want to keep updated of. eg. You live in London but your parents live in Ontario, so the GMT hand would be 5 hours back.