They were originally introduced in the early days of the previous NOTAM system when the server was housed at NATS Heathrow Communications Centre and a server or communications failure would have resulted in loss of access to data. In the early days of the previous system such failures did happen.
In practice of course it made little difference because most users had no idea that an alternate source was available.
The current system is delivered from an EAD server in a secure datacentre. The fallback position is a duplicate standby server in another secure datacentre (IIRC they are in Germany and Austria) that will automatically take over in the event of a failure. The feeling is that the reliability of the current arrangement negates the need for standby briefs to be available from the NATS server.
The contingency briefs are here and an address is included for any comments you may wish to make on the proposal.
PS If you read the heading as "NATS proposal to withdraw incontinence briefs" you need help.
