Kulwin Park asked,
but why do you use the term "ON GUARD" ??
Kulwin, it's a monitored (or guarded ) frequency. If you need help the idea is that someone is always listening and will provide assistance. The older style ELT's, epirb, ELT's etc that worked on 121.5 (no longer monitored by Aussar) would also be picked up by passing aircraft and satelites as well as civil ATC. Aircraft will still recieve a 121.5 beacon if its set off. Thats why boaties have to be careful when getting rid of older ELT's. Military acft still montior 243.0 (some new acft don't have UHF) as does Mil ATC.
The point of transmitting "on guard" is to make others aware that you are deliberately using the guarded frequency. Its mean't to stop others jumping in and saying "hey your on the wrong freq" that beer baron mentioned in the first post.