What is the best reason for the Constitution creating a separation of powers?

The intent of separation of powers is to prevent the concentration of unchecked power and to provide for checks and balances, in which the powers of one branch of government is limited by the powers of another branch-to prevent abuses of power and avoid autocracy.

In my honest opinion , i’d say franklin d roosevelt was a president that impacted our nation , i say this because fdr is voted among top three best us presidents and he was the longest serving president by 3 terms , usually presidents only supposed to have two terms increments of 4 years , equivalent to 8 years total before election time. and he established the fdic , through the 1933 banking act and he established the us , social security system , during his presidency, and established a national minimum wage , through the fair labor standards act , fdr led the allied coalition to victory in wwii and he played a major role in the creation of the united nations.