Why does the united states purchase oil from other countries and the problem this sometimes causes

The oil produced in United States is light and low sulfur content and they do not have proper processing facilities, hence purchase it from other countries.

Explanation:

About 9.93 million barrels per day U.S purchase oil from 86 countries. Because the crude oil present is not homogeneous in nature. It is very light in nature with low sulfur content or may be high,this particular factor depends upon the method of the refining. Most of the oil are being exported by the Saudi Arabia and then comes Russia followed  by Iraq. Despite the collapse in price, U.S. dependence on foreign oil should continue to decrease.