Why was bugle blown by one of the cavalry men?

The bugle was blown to signal the fact that it was time to advance.

As we waited for the bugle calls the air was electric with anticipation. It passed between every horse and his rider, between horse and horse, between trooper and trooper. I felt inside me a surge of such excitement that I found it difficult to contain myself. Captain Nicholls was leading his troop and alongside him rode his friend Captain Jamie Stewart on a horse I had never seen before. He was a tall, shining black stallion. As we walked forward I glanced up at him and caught his eye. He seemed to acknowledge it briefly. The walk moved into a trot and then into a canter. I heard the bugles blow and caught sight of his sabre pointing over my right ear. Captain Nicholls leant forward in the saddle and urged me into a gallop.