The same reason that a playbook helps in sports. In a group activity there is an advantage to coordinated action. In order to coordinate action, people need to know what the others are doing at any given moment.
A formation means people on opposite ends of the battlefield know what the others are doing and can focus on their more local environment instead. And if a change in tactics is needed, being able to tell people to change to a formation is far faster than explaining specific new instructions to each individual.
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