Can you tell me how understanding mitosis help you to understand leukemia and all other types of cancer?

It really doesn't: That's like saying that understanding the chemistry of burning hydrocarbons helps you understand your automobile, or understanding Newton's equations for gravity helps me understand how my parachute works. All tumors involve inappropriate cell growth but there is much, much, much, much more to it and unraveling the marvels of mitotis itself have contributed little that works against cancer.