I called it a "proof of concept". A lot of creationists argue against the existence of transitional species by claiming that certain combinations they view as necessary are physically impossible in a viable animal. This animal is a demonstration that an organism can have adequate lungs and adequate gills at the same time, as well as the capability to effectively move on land despite being an organism that generally stays in the water. That's how it serves as a proof of concept.
There are quite a few animals like this living today, but I found this one to be interesting because I've seen it in pet stores, and never suspected it had lungs.
Also, there's no such thing as literal "proof" in science, that is a thing exclusive to math. However, the evidence to support a scientific conclusion can be so vast and reliable that people treat it as "proof".
Coincidentally, via looking them up more, I found an experiment that pressured them into living a mostly terrestrial life. It's a pretty fascinating experiment as well, because the fish raised that way developed a different body structure than the ones that weren't, and the changes helped them live that way even more.
How fish can learn to walk
How lifestyle options impacts evolution is not a topic often covered, so I find it very fascinating. Makes you wonder how much that can apply to humans.
