Music is a powerful thing. It can soothe you, will you to fight, give you strength, and etc. and etc. Yes, music has many purposes when it comes to artistic meaning and value. However, in the world of Equis, it has another meaning. During war, it is often that officers and the like need to keep their numbers in check and tell them how to form, march and where to. Music became a reliable way of helping to get these orders through during the chaos of battle. So musicians were often employed to keep up with units and drive them through the battle, with the soldiers listening for the specific tunes that the music players would play. There was no set instrument, though small drums are more often seen. Because of their importance, opposing armies more often then not will try and target the musicians to silence them.

However, on Equis, Battle Musicians aren't without their means of offense or defense. Because of the strange ways that the realms play with the mortal realm, music seems to be a way of channeling the realms. Battle Musicians can learn how to bend specific realms to their tune and use them to invigorate, heal, strengthen, or speed up their comrades. As well, they can use harsh notes and songs to kill or incapacitate enemy foes if they get into the thick of it. Which is why there are many instruments brought onto the battlefield, as there are more musicians and strengths to choose from. Because of their usefulness, every nation uses them in their armies and won't conduct battles without them while also keeping watchful eyes on the enemy Battle Musicians.

I am reminded of the aliens from the original Transformers cartoon who uses sound and music for everything

that and Lucio :)