Oil serves as a medium for heat transfer from the pan to the food that is partially or wholy submerged in it. Without a liquid the food would only touch the pan at a few points. Oil can be heated to a high temperature while water is limited to 100°C. The Maillard chemical reactions that makes food brown or caramelized happen above 140°. Industrially produced oils are purified to remove components that decompose at lower temperatures or under exposure to sunlight such as free fatty acids.
They are naturally very bad for cooking and easily go rancid so they go through extensive refining which involves high temperatures and some chemicals. The process removes heat-sensitive ingredients so whatever’s left doesn’t go smoking in high temperatures.
They are very unhealthy, though, despite all the marketing to the contrary. Better use Lard, ghee or tallow.
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