The chain is what keeps the process going.
One branch has the power to carry out an action, while another branch has the responsibility of making sure this power isn't abused.
The Bill of Rights contains the first 10 amendments of the constitution. These amendments still greatly affect how citizens of the U.S. can live.
Both checks and balances and Bill of Rights maintain the power of the U.S. government, providing reason to why it is still relevant after all these years.