Why were the colonies an important aspect of mercantilism?

Answer 1

The entire mercantilist system depended on colonial empires.
The mother country essentially got all the wealth of the colonies in return for social, economic, and political security. Without the colonies, the mercantilist system would die because of the fact that the colonies were half of the system.

Answer 2

As you haven't specified which colonies, I can only give you a general answer, that they were the source of trade. The mother country, supplied things that were not obtainable, and the colonies supplied some raw materials. Usually the balance of trade is on the side of the mother country. This makes it profitable.