AADConnect is great isn't it? It syncs all your info to the fluffy cloud.
Almost all of it.
When you run a sync, anytime after the initial sync, it does not sync on prem users passwords even if you have password sync enabled. I'm sure that there is a built in schedule to sync passwords but what if you have a user who logged into the tenant and was able to change their password and you need to sync it back to the on prem password right away? A full sync or delta sync or a kitchen sink is not going to help here.
Now you could have password write back enabled if you have Azure AD Premium but not everyone has the cash for that. What you can do though is reset that users password on prem. Doing this on prem will force a password sync to the tenant right then and there.