Roy just beat me to it re the French!
As for a separate frequency or using the one you are on, 121.5 has much wider coverage and is linked to all the gear for locating these guys by triangulation. A normal ATS frequency could only give a bearing and not fix you on that line. Neck fully extended here, any expert able to elaborate?
There are also several well documented cases of light aircraft ever descending to stay out of cloud and their cries for help only being heard by Big Metal Drivers who could relay. The common frequency possibly saved lives, GA pilot or not is irrelevant at that point in time.