Setting the policy to Enabled lets users have Google Chrome check whether usernames and passwords entered were part of a leak.
If the policy is set, users can't change it in Google Chrome. If not set, credential leak checking is allowed, but the user can turn it off.
This behavior will not trigger if Safe Browsing is disabled (either by policy or by the user). In order to force Safe Browsing on, use the SafeBrowsingEnabled policy or the SafeBrowsingProtectionLevel policy.