I believe the answer is: To rule on whether laws or executive actions violate the Constitution.
When a new law or executive actions is established, the judicial branch in the government would held the duty to crosscheck the new law/executive actions with the current constitution.
This help the american system of checks and balances because the crosscheck make it impossible for both the legislative and executive body to create laws that stray away from the core principles that written in the constitution.
The Judicial branch's role is to check<span> both the legislative and executive by declaring laws unconstitutional. Even though there is more to it this is the main role played by the Judicial Branch.</span>
