I don't understand why people say oil is useful for plastics and such as though that was fundamentally different from its use as fuel. In either case, the reason we use it is because it is a pre-existing store of energy. If you have an energy source, then you can make oil, plastics, and whatever else you want. I have, or had, a mug from a former employer that was made out of plant-based plastic. It appeared no different from any other.