It seems ranking are a waste of time because there are so many contexts within which to be viewed. Even ‘mat time’ is meaningless as a practitioner may have simply been rolling around on said mat, singing musical numbers. ‘Black belt’ rank is thus very misleading as is ’master’, ‘shihan’ or whatever.
Why do we bother?
Why do we have "high school diploma" when there's a difference between valedictorian and someone who graduated with a 1.0 GPA? Why do we have "college degree" when there's a difference between an associate's and a doctorate? Why do we have a post count on this site, when some people post a single sentence and other people post a whole chapter?
If you go to a Judo tournament, chances are people of the same kyu level have a similar experience level. If you go to a Taekwondo tournament, chances are most green belts are about the same level. It's not always the case, but it is enough times you can expect it.
It's really useful for individual schools. A belt will tell:
- The instructors where you are in the curriculum, so they know what to teach you and to what level of detail
- The students around where you are in the curriculum, so they know whether to look to you for guidance, if you are going to be good competition for them in sparring, or if they should be the one guiding you
- The prospects around how long you've been going, so they know how much to judge the school based on you
For #1, if you are a green belt in my Taekwondo class, then I'm going to assume you know the basic kicks and stances. I'm going to teach you the green belt form and advanced kicks, and I'm going to go into more depth with you on the basic kicks and earlier forms than I would have in those classes.
For #2, if you are a green belt in Taekwondo class, then the white and yellow belts will be looking up to you. They know you've already passed a test on their forms, so if they have questions, they might ask you. A red belt isn't going to ask you about the red belt form, because they know you're not there yet. But they might have some tips on your green belt form.
For #3, prospective students will probably see the belt order on your website or your welcome packet. They're not really going to judge you based on your white belts, because you've had very little time with them. But the higher the belt color (especially black belts) and they will be judging the school based on them.