Simplistic example. Kill the city, the factories in that zone rebuild much slower. If you hit them next, the strats on the fields in that zone rebuild much slower as well. Hit them next. Now, in this particular zone, the city and factories are all at 0%. Convoys, barges, and or trains in this zone are now having 0 effect. Plus, the guys blowing all this **** up are going to run out of targets, and they'll probably go after your convoys next, and start hunting resupply goons.
Without the convoys, and with no barracks intact on any fields in that area, there are no troops, no vehicle supplies, no field supplies... no way to restore the strats on the field without flying them in from the next zone. Long flights in a C47 through an active area rife with enemies (in situations like this, normally sadists like myself, letting the goon get close enough to his destination to see it, then shooting him down).
End result for the red people- if you're trying to run some sort of attack out of this zone, you're SOL. You have planes, guns, and gas, but no troops, no radar at the fields, no bombs, no rockets, can't see much, and can't resupply anything to bring it back up faster. Your fields have been porked completely, and you might as well go make a sandwich, or get used to furballing, because the offensive part of the war, for you, is over for 2 hours.
On your clipboard map, rightclick, and select Legend from the pulldown menu. This will show you all the symbols of the various components and factories, and then it's just a matter of spotting them on the map. They vary somewhat, but tend to be evenly distributed across most maps, I think. I haven't attacked a factory in ages, because the choo choo of doom doesn't like me.
As to timing and whatnot, I suppose it depends more on what you're trying to accomplish. A complete and total porking takes quite a bit of coordination, and a lot of ord on each target. Unless you've got a helluva lotta people, you're playing whackamole. Get one target down, and another comes back up, etc etc.
That help any?