In many fields, leaders are let to step down and give his position to new leaders after a periods of time. For example, the president in a country, the CEO in a company, a dean in a school. New leader can give the enterprises new idea, new method and can also bring new vogors into the company. 
In the history, we can see many tragedy taken place because of a long-time reign. Many leaders are fighting for the agression or serving for people at first, but after a long-time reign they become more and more sefish and will act more strategy for themseves. This resulted in centralization especially in politics. For example, Stalin. Stalin was fighting against agression for Soviet people. After some time, he began his personality cult and he also change art on behalf of revolution. These, actually lead Soviet to an end quickly. 
In addition to, the changing of leaders can alter the defect of the previous leader. Because, the new leader as a bystander before can find the previous leader's defect to make up. This will surely be good to the company. 
But be sure to watch out doom loop proposed by the Collins. He said that when some companies were getting into trouble , the expedient can be the best way. So the board of directors change the CEO frequently, to look for the best method which can solve the present problem. Kmart is one of the examples. because of the doom loop, they came to the end of bunkrupt.