All,
Why ??
Because that is the frequency that is monitored by all airline aircraft in cruise, most of the time, and the frequency on which you are most likely to get an immediate answer if you are at low level and you have a problem.
I have lost count of the times, from "on high" I have picked up somebody on 121.5, when all other frequencies have failed to get a contact. When they are IFR, and I am calling for ATC, why no answer to ATC on the area centre frequency?? Who knows, black hole. any number of reasons, turned the volume down to talk to somebody, deselected VHF 1 to make an HF call, and forgot to reselect.
121.5 is the one where you might get to hear an ELT first, and help somebody else.
The real point about the training package is that is it is asking pilots to think, and use the frequency that is most appropriate for where they are and what they are doing, rather than a cast iron set of rules that only fit where they touch.
And to be damned sure they are on the local CTAF/MBZ frequency when they are any where near an aerodrome, stay away from approach and departure areas when reasonably possible, use the AIP provisions to fly off track, and so on.
In short, fly smart, stay right out of the way of the RPT aircraft.
Tootle pip!!