One example is fetching data, often it is more effecient to request data in a certain way, but the app needs it represented differently, multiple reducers can respond to the same payload and take what they need.

Another example is url updates, I use state-driven-routing and when a url update happens many reducers need to respond to get the pieces of state they are concerned about.

Another example is a login status change, many reducers may need to update themselves if a user logs in our logs out.

It's the difference between having tons of highly specific 1:1 actions getting called from the action creator with their relationship to the root action obfuscated vs. having 1 action that many respond to with their relationship to that action clearly denoted.

To take it a bit further, it reminds me of refactoring vs adding another "if" statement - the former requires comprehensive understanding of the holistic purpose of the system with the dependent relationships properly defined (anti-fragile), whereas the latter is a temporary bandaid that can be implemented with less thought (and the sideeffects that come with it).

Middleware. A UI events logger for example. Or finishing a network request may need to close some part of tge Ui and also trigger a reload somewhere else.