A breach on one site effectively rends all the others using the same credentials vulnerable as well. Apple advised affected users to change their Apple ID password as soon as possible Apple responded by advising users to change their password. The SSO tool can reduce the password problem to a manageable size by giving you a single master key to unlock the vault of unique passwords it has generated for you for each site and not reuse existing credentials. This approach contains the damage of a breach on only that system and prevents the sort of cascading failure.”]
Source: https://securityintelligence.com/reduce-reuse-recycle-except-for-passwords/