Every person who starts training karate-do has different goals on mind but one of these goals is very common on everyone and is to become Black belt.  And they know that the only way to earn a black belt  is by working with discipline.

Physical work
Not all persons are born strong, quick and flexible but by working hard on Karate anyone can become strong and flexible and react quickly when it is necessary.  Physical and mental work is necessary for this pourpose and this can only be done if the practitioner works with discipline.  
Discipline to success

Life at the school, at work or at the sports demands discipline to succeed.  Great things do  not come free  everybody has to work hard to acquire their goals. Karate helps  to create the sense of autodiscipline in the practitioner who will face any aspect in their lives  with the same spirit.
The Black Belt
When, after several years of training the person who earns a black belt becomes a different person and is conscient of his or her personality.  The black belt symbolizes perseverance, hard work, courage and discipline. The Black Belt knows his internal and external power and will use them only in extreme cases and never to start a fight.  A Black Belt learns to be humble.