It's complicated.

A battalion's first volley often was carefully husbanded; the mens' pieces were carefully loaded, had sharp flints, and could be better aimed without the thick white smoke choking the battlefield. Employed at the right moment, it could have devastating effect; not only would the physical damage of a carefully prepared volley be impressive, but the moral impact of seeing dozens or even hundreds of men fall in an instant can't be underestimated. Delivering a single volley was especially important when the battalion was facing charging cavalry, as they were likely to only get one shot before they were in cold steel range. A massed volley was more likely to unnerve the riders from charging home, and this hesitation would be felt by their horses.

That said, the common stereotype of 18th/19th century warfare in which all fire is delivered in tight, controlled volleys (stereotyped in the likes of Monty Python as enemies literally taking turns*) is not something that often manifested on real battlefields. Even in the Prussian service, famed for being the most disciplined and strict in Europe, their method of platoon firing was almost always seen to dissolve into uncontrolled firing at will in a matter of moments. Trained to load their pieces by rote, men found psychological comfort in going through the routine of firing and loading, without necessarily caring about combat effectiveness. Firing at will was recognized to produce greater execution over time, but it was also wasteful of ammunition.

During the era of the Napoleonic Wars and the French Revolution, armies began to incorporate more skirmishing into their fighting tactics. This meant large portions of the infantry would be deployed in open order, with large gaps between each man. Each man was allowed to load, aim, and fire at his own pace. Broadly, they came to believe this was the best method for delivering fire; its weaknesses were its vulnerability to cavalry and bayonet charges, as well as its difficulty in carrying out assaults.

*There's a famous exchange from the Battle of Fontenoy 1745 where the British Brigade of Guards called out to the French Guards, inviting them to take the first volley; the French replied that they could have it. Eventually, the French fired first; while they were reloading, the British marched up to close range and hit them with a devastating volley, killing or wounding 19 officers and more than 600 men. They then delivered a swift bayonet charge and drove off the French.