Final answer:
The preparatory command in a drill command is used to alert the unit that a command is about to be given, preparing them for the execution command that follows.
Step-by-step explanation:
The purpose of the preparatory command in a drill command is To alert the unit that a command is about to be given. Its primary role is not to initiate the execution of the movement, indicate the desired end result, or provide additional information about the movement, but to prepare the individuals for the execution command that will follow. For example, in the command 'Forward, march,' 'Forward' is the preparatory command that alerts the unit, while 'march' is the execution command that follows.