IIRC, these were implemented in earlier versions of ARMA.

Examples of these being useful:

"Fire on my mark" - particularly when setting up ambushes, which often benefit from precise timing. That way, once the squad leader opens fire, they do not need to spend a couple additional seconds to issue a separate order for the squad to open up.

"Return fire" - same thing when sneaking up on an enemy or getting ambushed yourself. Not having to issue a separate order saves a couple of seconds of reaction time.

To cut off the objections of the "but that's why commanding a firefight is difficult" ilk: this is exactly the reason such ROEs are used - to allow the soldiers to react to a rapidly changing, time-critical situation without needing to receive orders.