In the United States, the distribution of power in government is laid out in the Constitution, which delegates power to three branches: Executive, Legislative and Judicial. Other countries have varying forms of distribution of power.
Distributing power in government ensures a system of checks and balances, so that no one person or group has too much power. In the U.S., each branch of government has oversight of the other two branches. The President can veto congressional laws, and Congress has oversight on presidential appointments and has the ability to impeach the President. The Supreme Court can overturn laws, while members of the Supreme Court are appointed by the President and confirmed by the Senate.