Most things in life are grey. As human beings we are susceptible to making wrong decisions when things are grey. For example, I should only have a small piece of the brownie. Yeah, this looks like a small piece. But come on, one more piece wouldn't hurt me right. It's been 30 minutes since the last piece - surely I can have one more small piece, right?
The power of black and white is enormous. If we set up rules which make an action clearly right, or clearly wrong, we are much more likely to choose the right path. For example, I am following a diet called the Four Hour Body Diet. On 6 out of 7 days, I am allowed to have no carbs - essentially, no bread, pita, rice, ice cream, brownie etc. Just proteins and fat. The first 2 days of the diet are hard - damn I want that pizza now! But now it becomes clear what I can and cannot do - and I hardly ever go wrong. For six days a week, I have no pizza. Or creme brulee. It's as simple as creating the rule. That's the power of black and white.