Side arms refers to bayonets, and wearing them was considered an "honourable distinction" of the military profession and depriving a soldier from wearing a side arm was seen as a punishment. The soldier was taught to be "proud and jealous" of his side arm. Perhaps making them get their side arms was to remind them of their "honor." (I still don't see it, but it was different times.)
See The Spectator, vol. 8 p. 634