When programming, it is sometime useful to have an arbitrary value for a something that is available anywhere in the code, regardless of the current scope; essentially a default value. Here, setting an attribute value, does the same. The exact attribute name or the location if the note storing the value isn't important, as long as having made a choice the user refers to the name/location consistently.
Since the advent of the attribute() operator in v9.5.0+, another method is simply to make a user attribute per desired global variable and set its default, altering if needs be via attribute("AttributeName")["default"].
More common practice is designate a single note, often at the outline root as a configuration or 'config' note and store/alter global values there. By the same token, it is possible to use one attribute for several globals using a different note, but the same attribute, to hold each value.
There is no correct way. With the advent of attribute() adding a suitably named/typed attribute is the cleanest method as there are not locations to remember for getting/setting the attribute. In contrast, if globals might need to be copied from one TBX document to another, e.g. to set up a particular workflow, them a config note(s) is possibly an easier approach. This aspect of Tinderbox is still evolving so what is best/safest/least effort may change over time.