Answer:
To allow the national government to efficiently run the country.
Step-by-step explanation:
There are only 17 pages used to write the united states constitution. It is impossible to specifically mentioned every single power and duty of the government officials to run a country within those pages.
So, they created the necessary and proper clause.
The constitution only listed specific duty and power of the government in general term. The necessary and proper clause make it possible for the government to do what it deemed as necessary to execute the duties that mentioned in that limited general term. This allow the government to become more flexible in executing their duty and allowing national government to efficiently run the country.