Why was the Constitution necessary?

The original form of government that the American Colonies chose after independence was defined by the Articles of Confederation.

The AoC proved to be unworkable, with a central (federal) government too weak to be effective. By "too weak to be effective", I mean that it was soon recognized that the functions which citizens expected out of their federal government were not being met: it was unable to provide a national defense, an effective national currency, nor resolve inter-state disputes.

The Constitution was designed to solve the problems that the AoC exhibited. So, which it may not have been "strictly necessary", without the Constitution providing the glue to form a cohesive and functioning nation, it is highly likely that the early United States would either have disintegrated into smaller associations of states, or been re-absorbed back into the British Empire.

So, from my perspective, the Constitution was necessary to allow for the continued survival of the United States as a single nation.