Bit of an old topic but I can provide some insight.
On one of my instances we have a lot of users who have topics with frequent replies and don’t want to track them and set them to normal so they stay off Unread but will still show on Unseen.
We use it when we post in-topic moderator notes to make sure all participants in the thread see it. We upped the max number of users to notify to 50. We rarely have threads with that many participants but the ones where our mods tend to need to use it do.
I should also add we have it restricted to staff users to prevent it being abused. 