Your argument has a point but we are not talking about just fuels. You are right in that the energy would be most efficiently used to avoid extraction of more fossil fuel but..

In this case we are reusing plastic produced from Oil rather than discarding it. We have a large stock of plastic that we have no use for. Do we bury it, burn it (releasing CO2) or can we reuse it?

Re using this as a substitute for fuels or plastic feedstocks that have no ready sustainable replacement reduces the net amount of carbon emitted. Ideally we would have substitutes for every application but we dont yet (Jet fuel being a good example but we also dont have good substitutes for many plastics either) so these provide more pathways for non-polluting energy to be added to the mix.