Communications media exist to confirm the prejudices of their audience.
This law was invented to explain a paradox that troubled me for a long time: why do most communications channels spend most of their 'bits' conveying 'information' already known to the receiver.
In information theory, a bit of information is defined to be the distinction between two equally likely possible states, where the actual state is unknown to the receiver, but known to the sender. Sending a bit of information from the sender to the receiver allows the receiver to reach the same state of knowledge as the sender. If the information is actually known to the receiver already, it is, of course, unnecessary to transmit anything to ensure that the receiver knows what the sender knows.
The paradox appears when we look at any of the dominant communications media, such as television broadcast news. Stories are repeated over and over again, commentators make the same, entirely predictable comments, etc. When you think about it, it's quite easy for any member of an audience to simulate or satirize the content on almost any communications medium. If so, why do we need to waste the expensive bandwidth to deliver to the audience what it already knows is coming?
Thus we must look beyond information theory to explain why we need all those television bits, or radio bits, or newspaper bits, etc.
The law is my best shot at an explanation.