What is the purpose of separation of powers?

Answer 1

Some countries that do not want one person or group to have all of the power, so they created branches of government such as legislative, judicial, and executive. Each branch of government is limited by a national constitution.

Answer 2

to prevent the misuse of power
In England, the King traditionally held most of the power of government executive, legislative and judicial power, along with a parliament made of up a powerful House of Lords (constituted of landholders who inherited their membership) and a weaker elected House of Commons. (Note that although the House of Commons in the modern United Kingdom now holds almost of the legislative power and the Prime Minister the executive power, this evolved in the 19th and 20th centuries and was not the case at all in 1789).

When the US Constitution was written in 1789, the drafters feared placing too much power in the hands of single President, going so far as to consider a three-person executive, before settling on our current system. They separated the power of making law (legislative power) from the power of enforcement and administration (executive power) in order to prevent any one man from taking on the characteristics of a king.

Although the high courts of many nations overlapped with the other branches of government (the House of Lords being the Supreme Court of Great Britain, for example), the drafters also saw wisdom in creating an independent judiciary, nominated by the President and ratified by the Senate.