What's the purpose of cooking oil?

Does oil cook food in a different way? Like the difference between frying and boiling?What's the purpose of cooking oil?
It is the same thing. You are essentially boiling something in oil when you fry it. Since oil has a much greater boiling point than water, you can cook things at a much higher temperature, which is why when frying something, it is finished cooking much sooner than if boiling in water. Also, because of the chemical properties of oil, it cooks things up crispy when breaded (think chicken strips or fish).