Malwarebytes Labs - Pieter Arntz
Credential stuffing relies on the re-use of passwords. Take this example: User of Site A uses the same email and password to login to Site B. Site A gets compromised and those login details are exposed. People with access to the credentials from Site A try them on Site B, often via automation, and gain access to the user’s account.
If the user had different passwords on Site A and Site B, the attacker would have been stopped before they got in to Site B. This is why we are continuously telling people to not reuse their passwords. If all your logins are hard to remember (and they should be), you can use a password manager to help you.