It's not exactly happening in a vacuum. Emails and usernames from previous dumps help people figure out passwords for new accounts when many people are reusing

It's not exactly happening in a vacuum. Emails and usernames from previous dumps help people figure out passwords for new accounts when many people are reusing the same silly password everywhere. For example someone else may have had dadada as a password and isuckat@passwords.com as their email, and their password was discovered via a separate breach on some other site. That other site used shit technology for securing people's passwords. isuckat@passwords.com was in the linkedin dump. People can probably guess that any hash for isuckat@passwords.com will probably be 'dadada'. Now based on this information figuring out the hash for other users also using 'dadada' as their password is simpler. Multiply by many thousands of people reusing the same stupid password on all their sites. In other words use a unique password and use a different unique password for each site and service you use. Use a password manager.