This is mentioned in other responses, but keep in mind that frequently, it's urban sprawl which endangered these animals in the first place.

We're not going to start closing up our cities to let animals come back. But we can take steps to make the environments around them more hospitable: bridges under highways, reserved areas for wildlife. Once these are in place, some reintroductions to kick-start the process makes more sense.

A lot of the animals being reintroduced were key parts of the ecosystem before they were eliminated in areas, and their absence can lead to unexpected imbalances. Reintroductions makes sense in this context. Or at least, introducing another animal which fills a similar niche.