True. A branch of government is charged with ensuring that Civil liabilities are preserved by accurately interpreting the constitutionality.
Executing a law means giving validity to a legal document by signing it from requisite people. The government works on the principle of checks and balances. Laws are implemented for everyone to follow.
But there should be someone to keep a check on whether the rules being properly working or not. And check whether the people are following it. In US Government, everyone has a specific role, everyone has specific tasks and it requires everyone's cooperation in functioning.
