The global tag is hardcoded into the system as checked. Because all clients need to have an Access Tag, the global tag is used as a catch-all in the event that an Access Tag is deleted and the clients attached to it have not been reassigned to other tags. The clients would be lost inside your system with no one having access to them.
Because every staff member can see contacts with the Global tag, general contacts that can be seen by all staff can be given the Global tag. An example would be if all staff need to see certain referral sources or a particular business.