Some people get material sent to them for their comments. The matter may be very important to the sender but is outside the reciever"s area of expertise. There has to be a simple way of indicating this.
There are also times when the reciever does not want to get into a discussion on an issue. This may be a matter of pressure of time. Code 5/1 is a simple and direct way of declining to show interest.

It could be said that code 5/1 is unnecessary since a simple failure to reply would serve the same purpose. But a person who has communicated something that is of importance to him or her probably deserves some sort or reply.