This is probably a stupid question, but I find the instruction manuals rather unhelpful as to the use (or should I say the importance) of HQ units. Perhaps someone can enlighten me as to their overall function and the downsides of higher HQs being out of command.
Supply, rally disrupted units, keeping subordinate units in command so they can effectively spot for the artillery. Units out of command spot for artillery at a reduced effective fire, IIRC.
Units in command seem to gather replacements at a reasonable clip. Out of command units do not recover.
Those are just some of the benefits of keeping your units organized and in command range of their HQ.
All the above features of front line HQs become less effective when the chain of command breaks down from above with higher HQ's going out of command. I consider that a communication problem since it is temporary and usually sorts out on it's own in a few turns. There is a cascade effect of higher HQs being our of command that makes the lower HQ's more susceptible to being out of command.
Front line HQs which then fail the command check, cannot help prevent their own combat units form disrupting making the entire organization for that HQ more brittle.
Dog Soldier
Units operating outside the command range of their HQ's ("detached") can't recover from Low Ammo and Low Fuel status as those require a range check to the HQ. They can still pass the ammo test if they're detached if they pass the first test based on the local supply level. They just can't recover from Low Ammo status whilst being detached.
Detached units spot for indirect fire at half value.
Detached units receive 1/4 replacements through the replacement mechanic, but as far as I can tell the manual doesn't mention the effect of being detached on the recovery mechanic.
I try to keep all units comfortably within HQ range, as there are benefits to HQ's being close to their units, but in most cases infantry formations temporarily being out of command range won't be lead to disaster if they're B quality or above. Mobile units, or anything using fuel, is ideally always within the command range of its HQ.
The user manual specifies the effect of HQ's on various mechanics in the paragraphs on those mechanics.
As the guys say, whenever possible keep your units within command range of their HQ and each HQ above that in range of each other.
The effects of being out of command are quite subtle and you can be fooled into thinking the command aspect of the game is not that important, but it is so pay attention............
In the First World War Campaigns titles, units also drop one morale level when outside HQ range at the start of a turn.