The possibility to ignore handling of certain messages by setting a DSN like null://to disable handling those messages altogether.

Example
For instance, during tests I might not want to test that the message is stored, e.g. due to PHP amqp extension not being available but still want to test everything up-to the message being stored. The easiest way would be to set Null transport.
Of course, this is possible using the enqueue messenger adapter but only installing it and the enqueue bundle for a simple null transport is a bit unnecessary.