Coordinated volleys have a better effect than individual fire, although if a firefight went on for more than a few volleys, the unit might go to individual fire by default, as individual variation in loading time comes in and each man is frantically trying to fire as fast as he can. More can always be said on the matter if anyone would like to expand further on the little quirks of Early Modern warfare; for the meantime, I commend to your attention the appropriate section of the FAQ, specifically the headers 'Why did European armies use Linear Tactics?' and 'Mechanics of Linear Tactics'.

Adding onto that section, a few more posts:

• u/StoryWonker has an overview of the relevant factors;
• u/NotAWittyFucker also examines some of the relevant practicalities;
• and u/dandan_noodles looks at the dimensions of combat.

Note that these answers usually assume line infantry formed up in formation, as this tends to be the subject of most questions, but this is not the only manner of infantry engagement in the period. The American Revolution and the Napoleonic Wars are particularly notable for seeing a lot more light infantry action.