why is volley firing more accurate/better then everyone shooting at their own pace? Is it so you can do some sort of efficient timing of time between shots or what? how is 100 arrows/bullets at once more accurate then 100 shots on their own at the same target?

warsofasoiaf:

One big reason was morale. It took a lot of guts to advance against a line firing a hundred muskets in a line simultaneously, and morale was critical in warfare. The effect causes doubt, fear, as does a dozen people in the formation all falling over at once as opposed to one every so often. If you can break the spirit of the enemy, you can win the battle even if outnumbered. Units of troops are more effective than an organized mass, and breaking the formation often meant breaking the morale. It’s one of the reasons why professional troops were so valuable, their discipline and training kept their coherent. Muskets are largely inaccurate anyway, but a good volley toward a large body of troops has great visual effect. 

There’s also a benefit to unit cohesion, which is a less tangible thing. Training your units to fire and move together helps maintain formation and provides a psychological comfort to a soldier, which helps tie into morale as mentioned above.

Thanks for the question, Anon.

SomethingLikeALawyer, Hand of the King

It’s also about breaking up cohesion by slowing down the enemy. If one or two men die here or there, the formation can keep marching in step with only a few people having to step over a body. If a dozen men fall, then that’s a whole line that gets slowed down, and if you can rotate volleys effectively, you can slow down an advancing enemy to the point where their attack loses momentum.

Adding to your point about inaccuracy and limited range, muskets were also relatively slow firing, so as long as a formation kept moving at good speed, they only had to absorb a couple volleys before their attack hit home. However, if the formation got bogged down with corpses in the front ranks, this gave the defenders more time to fire, which increased their effectiveness.

A good (if somewhat exaggerated) example of this is what would happen to Napoleonic era French columns when they came up against British troops who had been drilled to fire and reload faster than the Continental norm. The French columns were very well-disciplined and were used to absorbing the punishment that the first few ranks might take while keeping up the momnetum necessary for the rest of the column to slam home, but faster volley fire meant that the columns often got bogged down and couldn’t close the distance before they were whittled down from a distance.