even today communication isn't instant and isn't free; the limited order per turn mechanic is maybe to much of an abstraction but think for example doing a synchronized turn at the Jutland battle:

from the admiral giving the order to his own ship flaggers to communicate to the next in line takes time, then the order gets flagged to the next in line ship, which has to rely to the ship behind etc. then the acknowledgement signal has to travel back and after that you can execute a coordinated turn

it all takes quite a while, and meanwhile you're best off not changing idea; even just cancelling the order introduce the risk to confuse and disarray your battle line

now, is it the order per turn the best system? probably not. Fields of fire had an interesting system that represent well modern era army organisation and disorganisation from units moving independently, but then again it's a very specific application, while strategy system or civilization games have much more ground to cover