does the globe have a technical function or is it just added for design?
Every single element of this watch is a functionality. It's the pinnacle of horlogery science and they make in very very limited numbers ranging from one hundred one time pieces world wide to less than 10. Here's a summary:
The GMT features a 2nd time zone GMT display, 24-second tourbillon, rotating globe with universal time display, 24 time zone worldtime display, summer time indicator, worldtime disc with summer time zones, day-and-night indicator, power reserve display, hours and minutes display, small seconds indicator and a equatorial sapphire crystal lateral window.