Firstly, it functions as a code, preventing communications from being intercepted.
Secondly, anthropologically it allows you to distinguish your group from the rest of the herd. There are plenty of real-world cultures that have an altered language or different language for the ‘upper’ class. It shows superiority over lesser castes.
Lastly, if they are trying to recreate something that they have read about, they may recognise the superiority of the Roman Army without having the skill to determine the specific elements that made them successful.