Why do some consider the supremacy clause to be the linchpin of the entire federal system?

The Supremacy Clause of the United States Constitution which is found in Article VI, Clause 2, creates that the Constitution, federal laws made pursuant to it, and treaties made under its power, constitute the supreme law of the land. It is called the "linchpin of the constitution" because it joins the National Government and the states into a single governmental unit, a federal government.