According to the article "The Famous Tax Included," what was the purpose of colonies in the eighteenth century?
I assume that you are referring the article from American Heritage magazine, 1961, that is in the link below. If so, the answer is that the purpose of colonies in the 18th century was to help the mother country. This is stated explicitly in the following quote from the article:
The eighteenth century regarded colonies as existing for the benefit of the mother country, with which, of course, the well-being of the colonies themselves was identified.
During the 18th century, essentially everyone believed in the idea of mercantilism. They believed that a country could only become rich by increasing its exports and reducing its imports. Having colonies was one way to help with this, and people believed that it was proper for a country to regulate its colonies’ trade in such a way as to help achieve its mercantilist goals.
It is important to note, however, that this article is not saying the colonists were abused. The quote above notes that the health of the colonies was identified with the mother country. What was good for one was good for both. More generally, the article argues that the colonists were not clearly being mistreated by the British and that British policies that seem outrageous to us today were really not seen that way by most people in the late 18th century.