“Battle Cry of Freedom” by James McPhearson covered this topic.

He wrote that military training and officer thinking for the first two years of the war was based on the musket. They learned through practical experience that rifles changed the game.

Muskets were highly inaccurate so their effective range was around 200 yards. An effective battlefield strategy was to march an overwhelming attacking force towards an enemy and then double time and rush the final couple hundred yards. Given how long it takes to reload, the enemy is only able to fire off one or two volleys.

Rifles were effective to around 1000 yards and very accurate at shorter ranges. So, the defending army was able to fire off many well placed shots before the enemy can take them by bayonet.

As a result, the strategic lessons learned and trained as part of the Napoleonic wars no longer applied. Defence now had the advantage in particularly when they entrench and build other types of fortifications.

With the rifle, the Civil War became more like WWI than it had in common with the Napoleonic wars. But officer training wasn’t up to speed. So, officers often sent in large head on attacks against fortified enemies with rifles and these turned out to be suicidal in nature.

The men learned through trial and error. Innovative generals who never went to West Point and read dusty old books made up their own strategies. Some clever men who did know old strategies rejected them and also wrote their own rules.

As a result, the war saw much less of the traditional formations especially as time progressed.

Because, believe it or not, it was the most effective means of controlling your army and delivering gunfire. Orders were delivered by trumpets and drums and bugles and spoken commands, so soldiers would need to stay close together, to each other and their commanders. Centuries of firearm doctrine and training manuals emphasized the importance of rate of fire over accuracy, because the smoke of gunpowder would quickly render the question of individual marksmanship irrelevant anyway - so formations focused on firing en masse in the general direction of the enemy, and putting their energies into reloading and firing as quickly as possible. While the musket now had rifling in the barrel, it was still a long firearm, meaning it could only be efficiently reloaded while standing. All of this comes together to mean that standing in line and delivering volleys was the most efficient method of keeping your army together and inflicting maximum casualties on the enemy.

These are all great explanations! Thanks everyone

For the most part yes. The reason comes from the French and Indian, Revolutionary, and 1812 wars when smooth bore muskets we're used. With a smooth bore, it's ALOT easier to hot a target that's 6ft tall land 150ft wide. Then rifles became more affordable and you could hit targets farther away. And rather than write up a completely new drill manual for the added range, they just used the old ones from the past. So except for a few skirmishes during battles (Antietam cornfield, Gettysburg round top) and what not, fighting was done rank and file. The most impressive use of rank and file was during Fredericksburg in 1862. Confederates lined up 300 guys wide and 4 guys deep behind the wall at Marey's Heights, guy in front fired while the the behind him reloaded and passed the weapon up. Devastatingly effective.