The reason for the reostat is that a stove can have too much draft, so some people put a barometric damper between the stove and the powervent, some people use a reostat to slow down the powervent. The best way is probably to combine the two with a manometer..
If you have too much draft, you just suck the heat out of the stove. If you have too little draft, you will have coal exhaust gasses escaping into your room. So it is important to have the correct amount of draft.
We have had many new members come on the forum looking to cure either burning too much coal or not getting enough heat out of their stove, furnace or boiler. Almost always the problem is too much draft pulling all the heat up the chimney or out the powerventer..
Most stove manufacturers recommend a barometric damper and checking the draft with a guage..
Hope this makes sense.. Greg L