Hi Big Beam, think about consistancy. You adjust your air intake to your furnace to give a certain amount of air for a given amount of draft.. If the draft keeps changing, how can you give a consistant amount of air to the fire?? An anology would be to try to keep your car/truck's speed constant with the throttle pedal, but the engine's power kept changing independant of the throttle pedal.
You want a consistant amount of draft, so you can give a consistant amount of oxygen [combustion air] to the fire.
Here are the two most agravating situations I use for illustration:
At 9PM, the wind is howling, the weather is cold,[very strong draft] and you set your air intake at a setting to keep the fire from burning too fast with the strong draft. AT 3AM, the wind drops, the draft drops, and the amount of air to the fire is reduced.. the heat is too low, and you wake up to a cool house, or maybe enven a cold house because the fire went out for lack of air.
At 9PM the wind is calm, the weather is mild, lets say 35*, so the chimney draft is a bit lower than average.. you set your draft to the fire to keep the fire going, because if you reduce it too far, the fire will go out without a decent draft.. At 3 AM, a cold front comes through, the wind picks up, the outside temp drops to 20* and the available draft in the chimney increases drasticly,, The furnace has the air set for low draft, so the added draft pulls a lot, too much, air through the fire, the fire burns too hot/too fast.. So you either get awakened by an over heated house at 5AM, or at 7AM you wake up to a cold house because the fire burn out, too fast.
With a Baro hooked up, you would have roughly the same draft all the time, [except when you have very little draft to control]. Your air settings will be much more consistant, more inline with heat output desired, rather than compensation for the weather and draft..
Consistancy is what it's all about. Big fireboxes like your Hot Blast are particularly sensitive to too little or too much air, that is a big pile of coal you are trying to control.. you need as much consistancy as you can get.
How much a baro appears to work is not the issue,, if it is set to control draft at say .05", but the chimney due to current weather is only pulling .04", the flapper will not move.. but if it is a gusty cold day, the flapper would be working quite a bit to keep the draft over the fire consistant. Without the baro, the air getting pulled through the fire would vary from slight to way too much..
Hope this helps.. Greg L