Ranging
In a drill laid down in 1757 the Number One, who laid his gun and corrected its fire, ensured the first round fell short. He then increased elevation until he obtained a hit. Such shooting is frowned upon today, but during the smooth-bore era once the target had been indicated by the BC the Number One was on his own. There was no OP, so if the first rounds were plus the chances were they would be unobserved, and therefore wasted.