As mentioned in the below comment, the author buried the probable lede: most of the time transmission is a placeholder to discourage others from using the band. During exceptional circumstances (as defined by the station's operator), it will transmit coded messages to global/regional operatives. E.g., to coordinate espionage when conventional communications are offline.
That constant sound is also useful for the spies. Without that it would be hard to know you have tuned on the right frequency (I'd assume they would use some pretty simple and low tech radios).
They would use commercially avaliable radios in the place they live to deter suspicion.
If you read the article it kind of gives it's most probably reason for it's existence away.