Here’s an article summarizing the findings of the most headline-grabbing medical study of the ISS program, the Twins Study in which Scott Kelly spent a year in space while his brother Mark remained on Earth; both were subjected to exhaustive testing, and the article briefly covers each investigation. One that I found particularly interesting: Scott’s telomeres grew significantly longer while he was in space!
https://www.nasa.gov/feature/nasa-twins-study-investigators-...
Most astronauts spend six months on the station, and obviously most of them don’t have a twin, but they all add data to our knowledge of how space affects humans. We have learned a lot about how to counter muscle atrophy, and we have a better understanding of how the eyes and bones are degraded by prolonged weightlessness.
NASA loves to talk about ways that space research has improved life on earth (just google it, they’ll shout it at you), but even if you could find a way to do all that other research without a crewed space station, you can’t accumulate six astronaut-years of human medical data in space every single year for decades on end unless you’ve got a place in space to put a bunch of humans for a really long time, over and over again.
To me, that is what makes ISS a human treasure. Someday we will build a more cost-effective replacement on the basis of the lessons we’ve learned, but for now it’s the best we’ve got, and it’s quite good.