Self-discipline is the companion of will power. It endows the stamina to persevere in whatever one does. It bestows the ability to withstand hardships and difficulties, whether physical, emotional or mental. It grants the ability to reject immediate satisfaction for something better.
The human being is full of inner unconscious, or partly conscious, impulses. People sometimes say or do things they later regret saying or doing. On many occasions people do not think before they talk or act.
By developing these two powers, one becomes conscious of the inner subconscious impulses, and gains the ability to reject them when they are not for his/her own good.