Direct link to the AIP page mentioned above.
Conspicuity squawks are normally issued by non-radar units so that adjacent radar units can see who's talking to who. That way if an aircraft was to infringe controlled airspace, the radar unit will know who to call straight away rather then ringing round the units.
I have always understood the primary purpose of a conspicuity squawk to be to let those operating adjacent zones know who you are in radio contact with.
I'm not sure I agree entirely. Yes, they do allow for easier tracing of an infringing flight but that is not always possible (e.g. in the case of 7000) nor is it, I believe, their primary (ho hum!) purpose.
Without a transponder (or a transponder set to STBY) there is no SSR plot on the controller's situation display. This means that the aircraft is being tracked by the PSR only and can be lost for various reasons (e.g. clutter or poor coverage to name only two). Selecting the transponder to ON (ideally with ALT) allows the controller to see an SSR position symbol on the surveillance display. However, it will show up with whatever code is selected on the transponder. Obvisouly, some flights are allocated a unique, discrete code and confusion may occur if a second flight happens to randomly select the same code. (Remember, not all flights need be working an ATS unit so they may not be issued with a code themselves.)
So we need another code which allows you to select your transponder to ON and therefore be more conspicuous on the situation display. You also become more conspicuous to any aircraft fitted with ACAS. Hence conspicutiy codes - by far the most common being 7000. Some units find it useful to have their own conspicuity code and these codes do, indeed, serve the purpose you both state, although looking through the AIP entry above many of them are surveillance equipped. Listening squawks can also be very useful here.
Doesn't even have to be validated or verified so be careful.
Again, I refer to the AIP link above, which states: "
Controllers are reminded that codes annotated with * are used for conspicuity, co-ordination or special purposes and the Mode A and associated Mode C pressure-altitude reporting data must be considered unvalidated and unverified." So it's not that they don't have to be validated and verified, it is that controllers must not treat them as such (even, in my understanding, if they have issued them and undergone the procedures to validate and verify). That is not really a pilot problem though and, being honest, I suspect many controllers would apply a more liberal, if incorrect, interpretation of the rules on this.