The necessary and proper clause has been one of the most important influences on the nature of American federalism. It has been interpreted in such a way as to allow the federal government to do almost whatever it wants in terms of making law.
The Constitution, of course, includes enumerated powers that are supposed to be a list of all the things the federal government is allowed to do. It also contains the 10th Amendment, which reserves to the states any powers that are not explicitly given to the federal government or denied to the states. Both of these parts of the Constitution would seem to limit the actions of the federal government.
However, the necessary and proper clause has been interpreted in ways that allow it to override the other parts of the Constitution mentioned above. Through this clause, the federal government has been allowed to make law in any area that can be connected to any of its enumerated powers. Since one of those powers is to provide for the "general welfare" of the people, almost anything the federal government does can be linked to its enumerated powers. For this reason, the necessary and proper clause has had a huge impact on the nature of American federalism.