This allows to create direct exchange as oppose to topic one

This means that you can now run multiple carbon-relays which can
essentially behave as "workers" when using direct exchange.
These workers are reading in metrics from the direct exchange
and distribute them to carbon-caches or other relays based
on relay method - there is no binding key necessary.
Previously you could only distribute based on binding/routing key
so you were not able to have true relay WORKERS which will not
generate duplicate writes on your drive. With topic exchange
and default binding key '#' you'd always get duplicate writes.