A dominant needs self discipline in order to consistently maintain his./her dominance within their relationship. It takes energy to use the control given you by a submissive. It takes self control not to abuse that control given you. Self discipline is part of what makes the difference between domination and abuse.  

A dominant needs self discipline to remain calm enough that he/she can clearly make decisions which affect someone else's life in a well informed manner. A person who has trouble making simple decisions for him/herself will have a difficult time being able to make decisions which govern someone else's life. A submissve looks to his/her dominant for stability and support, if the dominant is constantly in a state of emotional overload, the submissive will not be able to rely upon him/her. 

A dominant needs self discipline to exercise patience in learning to use the various implements of the lifestyle in a safe and knowledgeable manner. If a dominant refuses to exercise this self control and learn how to use the toys properly, he/she is then unable to consider themselves a safe dominant. Nothing is ever 100% safe and mistakes do happen, but they are less apt to occur with a dominant who has sufficient self control as to not do something he/she does not have knowledge of. 

A dominant needs the self discipline to remain consistent within the relationship. Therefor increasing the trust the submissive has in him/her and making it possible for the submissive to view him./her as worthy of their submission. A dominant who lacks the discipline to enfoce the rules he/she has set on the relationship, will soon find themselves with an unhappy and possibly rebellious submissive on their hands, if not a submissive demanding relase.  

A dominant needs the self discipline to remain physically in control of their actions no matter what emotion may be coursing through them. It requires self discipline to not strike out in a fit of rage when a submissive has displeased. It requires self discipline to not allow one's "love" for their submissive to interfere with enforcing the rules.  

Dominants often instill self discipline in their submissives by training them to speak more politely, control their orgasms, attain certain postures and thigns along those lines. Rarely is it discussed about how much self discipline a dominant must have as well. As you can see, self discipline is an important part of being a dominant since quite simply, without control over themselves, a dominant will be unable to control another.