Channels are primitive units where you can put any message and subscribers will receive all messages flowing through it. You will mostly see them in scenarios where a single event type goes through it, although that is not mandatory. The imperfect analogy would be a coffee vending machine that will produce coffee to whoever is using it.
Use them when you have a single event type used in a reduced scope.
Brokers correspond to a hub-spoke architecture. You will see them in scenarios where events are put for everyone disposal and it is up to consumers to pick the correct ones. The imperfect analogy would be an airport's baggage carousel, that produces baggage and it is up to users to pick the ones they are interested in.
Use them when you are working at a scope where there are many different event categories and your consumers may vary in time.
It is possible that some of the confusion comes from the fact that brokers might be seen as channels with added filters, but that is an implementation detail. It would be perfectly fine to have a broker implementation that uses no channels (let's say someone builds a broker that abstract Mulesoft or Biztalk).