It is the chain of command.
The commanding officer (CO) gives the command to fire to his adjutant, his adjutant tells the officer of the watch who passes it to the senior ranking NCO who then commands the men to fire. However, this is when the command is volley fire. The command volley fire is given so that most damage can be done at range whilst keeping the line or square. Volley fire means that only certain lines fire i.e. first rank volley fire, second rank volley fire. This gives the line that has just fired the chance to reload. Once the command fire at will is given then each man can fire again once they have reloaded without command.
It has nthing to do with tradition but rather to keep discipline in the midst of battle.
Oh and Captain_Augustus_McCrae shut up you wanka

British custom