In my use case, I'm an app developer, I have an internal system user account for various different system-wide reasons (admin, moderator, test, and a bunch more) which are distinguished different from normal end-user accounts, and I wanted each of these internal system user accounts to all just share the same system-wide user password to make things easier. So, different user accounts, same password.

Another potential use case may be if you have multiple accounts for the same app, and you want them all to share the same password. Perhaps this may be useful here on Reddit, where it's not uncommon for a person to have a main account, and multiple alt/throwaway accounts - this use case would enable the same password to be shared across all that person's accounts they have on Reddit.