I'll bet most here will say they are best used for artillery far to the rear,and i would agree.
Some artillery units have low ammo loads and fire more than one round per turn and will run out soon.
So having an ammo truck close by to artillery parks is essential ,I always save some support points in order to buy as much as i can.
But,if your not as much into "large" battles and/or artillery,kept far back as possible in cover,
but not too far they can be very useful as well,replenishing hardfighting undamaged armor.
Not as useful for infantry as usually when they run low they are too shot-up to be effective, better to save their hides for the next battle.
If you are asking how they actually work as in resupply,
just move adjacent to or in the same hex.
If neither unit is too suppressed or performs any action for a turn resupply should occur sometimes partial it may take more than one turn.