CHC is the largest claims clearinghouse in the US; about 100m people's insurance claims go through there each year.
The hackers asked for a ransom ($35m iirc) to delete the data, which United Healthcare (who owns them, because of course they do) paid. But it seems that the collective negotiating on behalf of the actual hackers rug pulled, so the actual hackers didn't get paid.
This is more than simply medical records. It includes who is active duty military and their family. If you can map where soldiers are, you know roughly the size of different military bases. If you know which types of capabilities are operated out of which bases, you can get a pretty good look of how the US is allocating personnel.
It was crazy working on recovery from this at the time. It should have been front page news, but wasn't.