On many of the British Firing Squads there rifles were loaded then passed to the man taking part. some guns would have blanks in them and others live rounds so that they would never know if it was them who fired the fatal shot.
My info may be wrong, but I was always led to believe that the blank round was for the above reason, as there was always an element of doubt.Another alternative would be to relieve the men of the possibility of being charged with murder, should the firing squad been found not legal for some reason. Then the all firing squad members could claim to have fired the blank.
It is clear that the soldier who had the blank would know but who could prove it.
The old .303 blank still had quite a kick to it although the noise was slightly different, still with all the other guns going off would you really have noticed and it would leave the firing squad hoping it wasn't them that had live rounds.