Lets say a species is extinct. By trying to bring the species back, are we doing them any favor? Somehow sounds more like something that humans would do out of vanity than to serve any real purpose. On the other hand trying to save species that haven't gone extinct makes sense as you are trying to protect their way of life.
E.g. restoring mammoths would add a bulldozer that would trample trees in the north and bring the place back to its natural state of grasslands, that protects permafrost and CO2 stored in it