I’ve spent quite a bit of time as a penetration tester and one of the first things we do once we recover credentials is check for validity against online accounts known to be good for a given user. We do that because it simulates attackers and government operators alike. It’s a guarantee that free credentials will be abused in one manner or another when they’re available to government entities.
The obvious control for this is to maintain a unique password for each account but that’s not always feasible for users due to myriad conditions.