Well, if not used by us, then they just are. They have no other purpose but the one we assign to them. It's like with that whole "save the planet" thing. The planet doesn't need saving. The planet doesn't care. It's we who care about having a habitable and pleasant environment.

I probably didn't make it clear enough, but what I'm suggesting is to use available resources to create better world for us and every future generation. The argument is more like "god put oil on earth for us to bootstrap a sustainable technological civilization with it".

"Purpose" is a human construct, so yes, in as far as oil can have purpose, it is for us to burn. And if burning oil would result in a net increase in utility, then not doing so is indeed a waste.