How important is grading?
It is up to the individual whether they want to grade as long as the instructor is happy for them to attempt it. Students are encouraged to grade when they are ready.
Why does Kyokushinkai Karate use the grading system?
It is a way of structuring the training, for example, as you progress through the belts you tend to take on more responsibility for helping lower grades until in the black belt grades it is part of the syllabus to hold certificates in coaching and first aid, etc.
To be confirmed