Question

Why does the United States use sanctions as a tool of foreign policy?

Answer 1

Sanctions are a way of achieving foreign policy goals without resorting to military interventions.  An example would be refusing to trade with a country until it improves its human rights record.   The economic influence of imposing  trade sanctions against a country might be a way to move that country along toward a foreign policy goal important to the United States, such as the recognition of universal human rights.

Answer 2

The United States uses sanctions as a way to punish foreign countries. Sanctions are essentially economic punishments made by one country against another. These punishments can include trade restrictions, tariffs (taxes on imported goods), or trade barriers. By implementing a sanction, one country is hurting another country's economy and financial well being. 

In using these, the US hopes that the country who they punish will change whatever policy the US disagrees with.