Aptitude test

Grading is all about aptitude.

A new starter could not possibly offer an experienced student good partner practice. 
The new starter's body use is too crude and clumsy and they lack the skills to take a fall or strike without injury.

Belts serve to differentiate the syllabus.
They allow students to progress in a clear, standardised, focussed manner.
At each stage, a specific degree of aptitude is required.

A student may well be keen, but without the competence this amounts to nothing.Grading involves a regular series of aptitude tests, and these serve to chart progress through the curriculum.