There are at least two ways to answer this.
First, we can say that the Articles of Confederation were needed because a country needs to have a constitution. A country needs to have some set of rules that will say how it is to be governed. Perhaps it does not need it if it has been around for a long time and has been gradually developing its government, but in a new country, there has to be some sort of constitution so that everyone knows how the government will be set up.
Second, we can say that the Articles of Confederation were needed as a way to correct the problems of the British government. The colonists had felt that the British government was too centralized. They felt that they had little or no say in it. Therefore, it was necessary to create something like the Articles of Confederation with a very weak central government and a great deal of power left to the states. Such a constitution would have reassured the Americans that the new government would be better than the old one.