The U.S and oil,why does it keep on buying?

I've done some research, and i now know that the U.S has a lot of oil. So my question is, why does it keeping on importing oil from other countries and spending money on it? Is it because the U.S is trying to save its own oil, maybe for the near future? WHAT FOR THEN, Don't know?

Just need advice because it's related to an assignment for my class, thanks.
 
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