No one is really sure
the general concensus is that they are used to send secret messages to agents in the field. They take the string of numbers and decode them, but how much infor can you convey in repeating the same 5 numbers over and over?

Oh go on, click the link. It's not horses, I promise.
( sandettie light vessel automatic New Twitter - @bollocksreally, Fri 8 Apr 2011, 0:06, 3 replies)

I kept expecting the picture to change to something horrible
and then jump through my screen and rape my soul
( kappynitty doubled your entendre on, Fri 8 Apr 2011, 0:50, closed)

LOL
Ive been wary of clicking links to get that bloody "I love horses" thing
But seriously thtas a very odd thing
(insomniac-surfer oh go on then, just this once, Fri 8 Apr 2011, 1:44, closed)

If memory serves, the Czech Republic has been the only country to date that's openly confirmed its use of number stations in espionage, though that was only after their stations had been decomissioned.
And without the code, the message is completely meaningless - which is probably why the senders are not bothered about sending unencrypted transmissions that can be picked up by anyone in range. For all we know, the number of repetitions might be part of the code itself.

As a method of relaying information, number stations have a lot going for them. They can easily be broadcast from anonymous locations like industrial estates, they use analogue technology so they can't be electronically traced (unlike anything on the internet), and, notwithstanding the efforts of radio enthusiasts, they're hard to detect if you're not looking for them.
( The Mock TurtIe ™ --- Thinks you are a cunt, on, Fri 8 Apr 2011, 2:39, closed)

More importantly
They need nothing more complex than a shortwave radio to receive. Whilst it's probably fairly unusual to see a shortwave radio in somebody's luggage these days, prior to t'internets it was perfectly non-incriminating.
( multijoy, Sat 9 Apr 2011, 12:05, closed)