Besides what others said, oil is kind of like liquid thermal paste. It distributes heat evenly to the surface being cooked. Try browning broccoli wothout oil. The parts in contact with the pan will brown, but the parts not touching it will be uncooked.
Drop some oil in there, and the heat gets distributed evenly, and you start to brown it more evenly.
Adam ragusea has a good video on oil if I’m not being too clear.
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