It is codified bc unlike the UK's constitution, it is written down in a single document which sets out the structure and rules of governance in the US
Individuals and institutions that govern the US derive their authority from the Constitution
The Supremacy Clause, contained in Article VI, crucially sets out that the Constitution is 'higher law' than any other legislation passed in the US
This means that other legislation can be struck down by the judiciary if it doesn't abide by the Constitution