Every now and then it would be nice if you had a stable signal available, the frequency of which was known exactly. For example, to finally calibrate your frequency counter properly, or as a reference when working on radio frequency circuits, or — well, anything else for that matter, since a good reference will come in handy sooner or later.Not all that long ago this was quite a difficult problem. For simple hobby purposes you could just use an ordinary crystal — but no crystal oscillates at the exact frequency that is marked on its housing (although… years ago I bought, for not even 20 euros from a now long-vanished department store, a quartz watch the crystal of which happened to be ground so well that the magnitude of the error was about a minute per year) and in addition, the frequency is temperature sensitive.