There is news almost every week about hacker attacks where passwords were stolen. Even well-known companies (LinkedIn, Dropbox, Adobe, …) have already been among the “victims”.
Many (but not all) of these stolen passwords then turn up on the Darknet and are used to access other services.
About the website haveibeenpwned.com you can check if your own email address is affected by known hacks.
If your email address has been hacked and shows up in this database, it is important to use different passwords for all online services, email and company login.