It was for D & D Controller training..
ie. Using the DF equipment and aids to locate the practice pan and then deal with it.
The thing is that it is seriously annoying, especially at FL160. 121.5 is supposed to be an emergency frequency that crews shouldn't have to tune out...
I guess they first should train to find another training frequency that is not monitored by thousands of aircrafts around
. I do believe people are trained as well in other countries but obviously not on 121.5...
Of course I don't see any obligation and it's probably just a question of good manners just like not farting loud in a 5 stars restaurant.