It allowed the ability to command everyone by voice as well as keep track of them all; something very crucial to an army made up of a bunch of a blob of people who didn't want to be there. The former especially allowed them to coordinate fire; two lines alternating barrages every 15 seconds is far more impactful on the enemy state of mind and morale than a bunch of scattered pot shots.

Further by keeping them together it allowed them to act as a cohesive unit. This allowed them to use bayonets to directly fight cavalry (an action spread out light infantry by definition can not do effectively) and also engage other groups of infantry as a block of bayonets; an act similar to a phalanx charge which, by definition again, the larger, denser, more cohesive one will inevitably win out.

Ultimately these advantages far outweighed the cost of taking a marginally higher death toll by grouping up. Dave Grossman cites that even when rifles were being used in the Crimean War Frenchmen would still only hit about 1 out 7 times. In the ACW that number was more around 1 out of 50 times and those are still with rifled muskets. Smooth bores did not much better, especially early 18th century ones.