The Necessary and Proper Clause is the final item in the list of the powers given to Congress by the Constitution in Article I, Section 8. The previous items in the list are all very specific statements of duties to be fulfilled by Congress. Included in these duties were the establishment and maintaining of an army and of a navy, establishment of post offices, printing currency for the new nation, conducting trade with foreign nations, and so on. These were all fairly straightforward functions that could be begun immediately and continued into the future.
The Necessary and Proper Clause was written to give Congress the power
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 in the Government of the United States, or in any Department or Officer thereof.
The authors of the Constitution recognized that conditions would change as the years went by and that fulfilling the powers given to Congress might require laws that could not be predicted in 1787. The Elastic Clause allows the Constitution to "stretch" - to expand as needed to meet circumstances that the government or one of its departments may face in the future.