People experience decision fatigue. So leaders like Jobs and Obama once decided to wear the same thing every day and save their decision-making powers for more important matters.
Organizations are vulnerable to decision fatigue as well. So leaders make rules to simplify decision-making for their employees. When employees face situation A, they need to take action X.
Leaders must be mindful not to create rules that remove judgment completely. Rules of that nature destroy trust and can lead to harmful consequences. For example, a rule prohibiting employees from accepting gifts from suppliers will work well 99% of the times. But once in awhile it will insult a supplier and damage a key relationship. Employees must be trusted to make judgment calls.
Even Steve Jobs wore the occasional tuxedo.