When we talk about using unique passwords with every website and account, this is why.
If your password in this database of 26 billion records is in use on any other website, they will be logging in as you!
If it is the password you use to login to your computer, they are logging into your corporate network.
More than 80% of confirmed breaches are related to stolen, weak, or reused passwords.
If you need help keeping them all straight, consider using a Password Manager.