This phrase recurs throughout the Articles of Confederation. In fact, you almost never see "the United States" without that big ol' qualifier. The repeating (and repeating, and repeating) phrase emphasizes that the states only act as a single country when brought together in Congress.

It's sort of like calling someone an "acquaintance" rather than a friend; you want to make sure that no one thinks you're really close to that person. Also, in other texts of the time, the nation was often referred to as "the Confederation," not "the country."