Message Carriers are all supports, from very simple to very advanced ones that we can use to give our communications some kind of a brand. This helps recognising the source of the expressed message, topic, theme.
An example of a simple form of message carrier is a meme.
We need message carriers to get and sustain continued attention for a topic or theme. But we also need a message carrier to be recognised as a source of treated and presented topic(s)/theme(s).
Logos and Branding are one thing. A message carrier can be more than only a logo or a brand.