[Suhiir]

As a former FO and (later in my career) HQ unit weenie I'll give you my take on things.

As an FO your primary job is knowing what fire support is available and locating targets where it'll do the most good.
Dumping air burst rounds on opposition armor is not a good use of available assets if you have regular HE available (not that armor is a good target for artillery in the first place).
FOs "speak" the same language as cannon cockers and almost always have direct access (via radio) to the Fire Direction Center or specific battery. So they can make the call quicker and get FAR better first round accuracy (as a side note I have a "professional" objection to the amount of scatter fire has in WinSPMBT ... but let's not go there).

HQ's rarely know the specifics of what indirect assets are available. Their calls for fire usually get whatever is available at the moment because they need help NOW not when a better suited asset is available. HQ units rarely have direct access to firing batteries. They have to go thru their chain of command to make contact. And while the HQ folks may know the procedure for calling in and adjusting fire they don't "do it for a living".