Why is it important for the powers to be separated and for one person not to have all of the power to make decisions?

It is important because if one person had all the power, instead of what is called "separation of powers" then that ruling would most likely become a tyranny, because that ruler would feel powerful and reign during his/her jurisdiction and then that would be considered an absolute Dictatorship.

Think of it as a war start mostly because of fear or intimidation If The United State Was governed by a corrupted president who Had authority over everything and could overrule anyone that would be considered to much power that is why every decision he make has Congressional approval if he wanted to declare war because someone looked at him wrong he would have to have Congressional approval