Why it is impossible to allow the concentration of state power in the hands of one body. What are the consequences of this?

The concentration of state power in the hands of one body should not be allowed. Because a person in whose hands there is too much power will begin to abuse it, use it in the wrong place. Therefore, in democratic countries there is also a division of powers into the legislative (which comes up with and adopts new laws), the executive (which is responsible for the state budget, that is, what to spend it on) and the judicial (which is responsible for justice and order, it also monitors for the exercise of human rights).