In the words of W. Windham, author of "A Plan of Discipline for the Norfolk Militia",
It was therefore necessary to exercise the troops in loading quick, and firing together by the word of command: but as the awkwardness, carelessness, and rashness, of young soldiers, (if left to themselves) must occasion frequent accidents; and cause the loss of many of their own party, by the unskilful manner of using their firearms, especially in the hurry of an engagement; it became a matter of indispensable necessity to teach soldiers a uniform method of performing each action that was to be done with the musket; that they might all do it in the most expeditious and safe manner.
In order to effect this, it was necessary to analyse and reduce the compound motion of each action into the several simple motions that it was composed of: this made each action easier to be learned and remembered; and by teaching the soldiers to perform the simple motions in the same manner, and in the same time, making a pause between each, it rendered them exact in the performance of the whole action.
This is the origin of what is called the Manual Exercise; which, when it was once invented, (besides the real utility of it) made troops to show to such advantage, and their motions appear so regular and beautiful, that it soon was copied by other nations, and came into general use.