In my opinion, self discipline is obviously a trait every police officer in the force should possess due to some of the intense and stressful situations that can come out of wearing the badge. If you're unable to have hold of your own emotions and keep control, it would evidently lead to hasty, unthought out decisions, something that is clearly unwanted within the force. Moreover, if you lack self discipline it implies that you're unable to progress in overcoming weaknesses, which can cause for unwanted behavior such as not being able to take orders, or maintain a general brash attitude could lead to a bad public image and a bad reputation with colleagues.
If an officer does maintain self discipline, it is much more likely that that he/she will have a much better attitude to taking orders, keeping calm in the most stressful or difficult situations, and not let emotions come in the way of work that lies ahead of them. And generally if you're unable to keep emotions out of work, and view every situation as somewhat as an impartial spectator, you are much likely to work efficiently, and also take the best course of action for any given situation. An example would be sympathy. If you're sympathetic, you could possibly be inclined to do something foolish and unthought out, such as let someone off because you felt bad for them. Clearly, this attitude is not something wanted within the force, as you can't uphold the law if you turn a blind eye to any situation.