The importance isn't so much upon becoming really good at the sport, as much as it is to just be doing something after school. As a result of these after school practices, teachers are extremely busy during the day. Since, time that could normally be used for grading, lesson planning, etc. gets eaten up by their coaching and supervising time. Teachers here tend to be exhausted by the end of the day. In many ways exhaustion is a social tool in Japanese society.
Emphasis is placed on subservience to the group, depot being totally drained.
Operating in this public arena, where one is subject to constant scrutiny by one's colleagues also serves to keep individual creativity, new ideas, and egos in check.