Question

The Necessary and Proper Clause is important because

Answer 1

Also known as the "elastic clause," this clause is one of the most powerful in the Constitution. It allows the Government of the United States to "make all laws which shall be necessary and proper for carrying into execution the foregoing powers, and all other powers vested by this constitution." This has been used for all types of federal actions including requiring integration in the states.

Answer 2

if you're a progressive, it's important because it allows modern courts to read into the Constitution powers for Congress, the President, or the judiciary which aren't explicitly there, but which you believe the Necessary and Proper Clause implies. In essence, you say, well, the Framers WOULD have explicitly given this or that particular power to Congress, but the issue didn't arise in their day. However, we can read from the rest of the Constitution the general ideas about what they intended, and that lets us believe they would've meant for this power to be given to Congress (or the President), and the Necessary and Proper Clause tells us they intended us to therefore give it to them.

If you're a conservative, it's important because its interpretation (or misinterpretation, as you would put it) allows the progressive to work towards all kinds of outrageously unconstitutional expansion of government power, in response to the popular political fads of the day, and thereby makes a mockery of the whole idea of a constitutional republic. If the government can do whatever is popular at the moment, why bother having a constitution at all? Or why not just have a one sentence constitution: "Congress can do whatever it wants, and the President can, too, unless Congress disagrees, in which case the Supreme Court breaks the tie."

You might say it's the difference between a reformer and fundamentalist reading the Sacred Text, e.g. the Bible. The fundamentalist says the Sacred Text means exactly what it says, and argues there's no point to even having a Sacred Text unless it constrains modern thought, regardless of whether it happens to be popular at the moment. The reformer says nonsense, the point of a Sacred Text is just to establish some general ideas and principles (*be nice to animals", "don't steal") and the exact interpretation of how those principles should be in light of the events and situation of the moment.

One viewpoint prizes flexibility in meeting the specific situation of today, and fears the evils of inaction. The other prizes steadiness and being true to ideas that have stood the test of time, and fears the evils of misguided but momentarily popular action.