Training
I at lease once in a PPL course fail the radio on the student, ATC is briefed to expect the non radio arrival and I get to listen to the goings on as the radio magically only fails for the student.
It is a real confidence building excersise for the student to see the system on light signals kick into action and to see that a VFR radio failure is NOT a crisis.
However other pilots in the CCT sometimes seem to be able to make a non- radio aircraft into their own crisis, it's very abusing to listen to when your student is blissfully following the proper course of action totaly unaware of the crisis others are making of the situation.