The Founding Fathers created a system requiring judges appointed by the President to be confirmed by the Senate. This was done to prevent any branch of government from becoming too powerful.
There was a real concern when the Constitution was being written about individuals in the government or the government as a whole having too much power. This was part of the reason colonists fought for independence from Great Britain. To try to prevent this from occurring in the United States, the Founding Fathers created the concepts of separation of powers and checks and balances.
The separation of powers idea refers to giving each branch of government different jobs so no branch can do everything on its own. The system of checks and balances allows each branch to limit the power of the other branches. One way the legislative branch can limit the power of the executive branch is through mandatory Senate approval of the Presidents' judge appointments. This is happening at this moment with the Supreme Court. The President made an appointment, but that appointment won’t go into effect until the candidate is approved by the Senate.