Major One of the proposals for message format indications suggests a kind of convention using a parallel metadata topic tree. For example, a retained message on topic "$META/Fruit/Apples" could be used to indicate the format of messages on topic "Fruit/Apples". A client wishing to publish or subscribe on "Fruit/Apples" could first make a subscription on "$META/Fruit/Apples" to discover the message format. Using the existing pub/sub rules is inconvenient in this case.
If I want to use the same message format information for all fruity topics, I don't want to publish separate messages on each of "$META/Fruit/Apples", "$META/Fruit/Mangoes" and so on. If I publish on "$META/Fruit", it would not be sufficient to subscribe to "$META/Fruit/Apples" to discover the message format. What I want is a way to publish a message for all fruits.
Another example comes from the IoT domain. Let's say that I have connected lights subscribing to "lights/<light-id>/off" and each light turns off when it receives a message to its topic. If I want to turn off all lights at once, I could define another topic "lights/alloff" and get each light to make a second subscription. Alternatively, I could publish one message to all lights.