Why does the Canadian government impose duty on foreign purchases?

Duty rates vary greatly depending on the product and the country in which it was manufactured ... If you're having goods shipped to Canada from a U. S. online retailer, you will likely have to pay Canadian duty on the value of the goods, plus GST and PST or HST, depending on the province you are importing goods into.

Explanation:

U. S. tariffs on Canadian steel and aluminum

On May 31, 2018, the United States announced that tariffs of 25% on imports of Canadian steel and 10% on imports of Canadian aluminum would take effect on June 1, 2018.