If Canada chooses to declare its independence and agrees to the terms of the Articles of Confederation, it can join the union and become a fully sovereign state like the other thirteen states. This offer does not include any other colony but Canada, unless nine states agree to extend this offer to another colony.
Establishing both the means by which a new state could enter the "union" on equal footing, and an attempt for military security, Article 11 specifically targets one issue in a way that no other article does.
Annexing Canada and formally absorbing it into the folds of the United States would have increased the power of the U.S. tremendously. The inclusion of Canada in the union would significantly increase the U.S. resources of land, people, types of industry, and available ports. It would increase the tax base of Congress as well as contribute its valuable resources to the overall economic good of the U.S. Furthermore, the annexation of Canada would help to significantly eliminate the biggest threat to American Independence: the presence of Great Britain on the North American continent.
If Canada had overthrown British rule in the 1780s and joined the United States as a sovereign state, the British would have had no further holdings of land in North America. However, after the war, the British continued to violate the Treaty of Paris by maintaining forts in the western territory of the United States. The British controlled the Great Lakes, which bordered the U.S. and Canada and the St. Lawrence River, thereby giving them powerful control over trade in the interior of North America. The United States aimed to eliminate the presence of as many of its competitors as possible. Unfortunately, Canada had no interest in joining the United States and remained a British colony. The British presence north and west of the United States continued to be a problem, and eventually led to the War of 1812.
What this Article did accomplish, however, was to establish the precedent by which new states would be absorbed into the union. With this precedent, rather than representing a governing body with fixed limits, the United States would be able to expand and absorb sovereign states on an equal basis, instead of as colonies. This idea was later put into practice in Thomas Jefferson's Land Ordinance of 1784, which provided new states with the same right to self-governance and representation in congress as those enjoyed by older states.