Why is judicial review such an important power for the supreme court to have?

Judicial review is the system to look over the activities of the government or the policy making branch while passing the law after deliberating it in Senate. Judicial review is carried out by the apex court and it has all powers to invalidate the laws which appear to be unlawful or biased.

The Judicial review taken up by the court can also consider a mandate to be invalid if it doesn't pertain to the terms of the constitution.

This procedure of judicial review is the right given to the United States' Ultimate Court under the structure of Checks and Balances and equal delegation of Powers proclaimed in the constitution. The activities of both the branches are regulated by the judiciary through the judicial review and order them to stay within the confines of their authority.

Answer: Before Judicial Review the Judicial Branch wasn't nearly as powerful as the other two branches Executive and Legislative because the Judicial Branch. Judicial Review have the Judicial Branch the authority to monitor the other Branches through Checks and Balances.