Changing the password on different systems on different schedules discourages password reuse.
But it doesn't, it really doesn't. It just results in people buttonholing sysadmins in the corridor asking "when are you going to stop dicking around and implement SSO?". Not long after that, people just start ignoring security advice altogether.
You are right about reuse across multiple internal systems not being strongly discouraged.
Where it does discourage reuse is across multiple systems, and with websites. Making me change my corporate password every 90 days is an effective way to ensure that I don't use my current password across a large number of websites. Maybe I'd go to the effort of making my gmail password the same as my corporate password. The password on that random news site account I forgot about? No ways.
It's not a panacea, but password expiry does effectively limit the spread of passwords in many cases.
But it doesn't, it really doesn't. It just results in people buttonholing sysadmins in the corridor asking "when are you going to stop dicking around and implement SSO?". Not long after that, people just start ignoring security advice altogether.