It is basically the default place to put your macros, and make them available always.
When first starting out, this is a good place to put macros - they work and they always trigger like you expect.
After you discover that you want per-application macros, you typically start creating macro groups for each application, and limit that new macro group to that application and place application-specific macros in that group.
Later, if you have lots of macros, you may want to create macro groups for specific purposes, for example you might have a group for text expansion macros, or for application launching, or for opening web pages, or whatever. Or you may make macro groups to create custom palettes.
You may also find that some of those global macros end up conflicting with weird applications (Remote Desktops for example or games or whatever), so maybe you make a Mostly Global Macros, and move some of those macros in there, and exclude the problematic applications.
Personally, I have 60 enabled macro groups, and I still use the Global Macro Group, which has about a hundred macros, about 20 of which I still use, and the rest are just left over from something or other.