For the drum is the voyce of the Commander, the spurre of the valiant, and the heart of the coward; and by it they must receive their directions, when the roaring of the Canon, the clashing of Armes, the neighing of Horses, and other confused noise causeth that neither Captain, nor other Officers can be heard.
The drum was intended to carry the orders across a noisy battle field and set the cadence of the marches. Drum corps would be arranged throughout a regiment in order to pass on the orders, controlled by a Drum Major.
We are lucky that a number of tunes and calls have survived from the period, some still played in armies today. Music annotation was different in the 17th Century and tunes have been translated by different specialists. We play those same tunes today.
Drummer boys didn't come into use in battlefields until well after the English Civil War. Drummers in the period were mature men, likely well educated and of high birth. These men were literate and often multilingual, and were often used as messengers and to undertake parlays.