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).
