it regulates the stove much better with one, especially if you have a tall chimney, granted, some may not even need one, if you have a moderate draft and a short chimney, it can't create a lot draft, therefore, with or without, you would still have the same draft.
The warmer outside, the less draft you will have, that is why when it's really cold out, HOT AIR RISES much faster and therefore creates a lot of draft, if you didn't have a baro damper, it would suck all the more heat from your stove and make it burn much faster and waster more fuel.
My chimney is like like 30-35 thru the house and out the peek. I can close off my Baro and my Manometer will run up to almost -.08 - 1.0, when it gets really cold out, it can go even higher. The baro opens and regulates the stove back down to the setpoint of .04 or so. Keeping as much heat in the stove as possible. otherwsie it would be sucking more heat from the stove that I can use in the house....
I have never had any fumes come out the damper even on very low draft (less than .02) on a very warm day. Unless the pressure equalizes at the top and the bottom, it will always want to draw upwards creating a vacuum.http://en.wikipedia.org/wiki/Chimney#Ch ... t_or_draft