The reason we import oil is that the majority of our lower 48 production is not oil but condensate. IE it has an API gravity greater then 45°. Our refineries were designed and built at a time when the crude mix was primarily below 38°. Canadian crude from tar sands and such is mid to high teens, Saudi crude is mid 30's, Venezuelan crude is below 20°. We have to blend our condensate with imported oil stocks in order for them to be efficiently refined. Without these imported barrels our refineries would function at a significantly lower capacity. Additionally we have more refining capacity then we have need of so countries ship their oil here and we export refined product back to the global market. Some smaller western refiners built more recently are called crackers and can manage the light ends of most unconventional production (commonly referred to as shales but rarely actually shale formations) currently flowing in the lower 48. Additionally the lighter grades of condensate the makeup of the barrel does not distill into the same mix as the lower grades. This means is breaks more toward gas and less towards diesel. We started exporting unrefined products in 2016 during the Obama administration, after having banned the practice in 1975. We currently produce over 12 million bbl per day with estimates being almost 8 million of those bbl being condensate.