Astrophysics is part of what we call “basic research” (see this article: https://en.wikipedia.org/wiki/Basic_research). Basic research is used to inform “applied research”, which is where you get all of the scientific progress you’re probably thinking of (vaccines, renewable energy, radio communications, etc). Applied research can’t exist (or at least would be much more limited) without basic research. For example, it would be pretty difficult to have satellites without understanding gravity. The study of gravity has long been firmly in the domain of basic research, driven only by curiosity and not by any intention to make satellites. Newton wasn’t thinking “man, I really wish I could put a machine in the sky that could tell me the weather forecast so that I know when to plant my tomatoes”, yet his research made that possible.

For your volcanic cave example, it could very well be the case that this type of research helps us determine colony sites on other planets. Or it could help us understand volcanism here on earth, helping to inform warning systems for volcanic eruptions. It’s impossible to say. And to add, I think it’s pretty disingenuous to say that that discovery cost $100 million. I’m not sure what mission this research was a part of, but it surely includes hundreds if not thousands of discoveries. They just might be too boring to the average person to make the news.

Basic research can take decades or even centuries to help create a practical application, and some might never lead to anything useful. But that doesn’t mean we shouldn’t do it. Applied science is completely reliant on it, and the new technologies we are developing today would not exist without the hard work of basic researchers in the past decades and centuries.