It helps you determine what it will cost to heat, and what you should do to seal it up.
When Greening Homes did the blower door test on my 100-year-old house before we renovated it, they couldn't even get to 50 pascals of pressure, it was so leaky. It really was as if I had all the windows open. We also ran around the house with a thermal camera and that was just as shocking; I might as well have been raising my family in a tent.
These tools give you so much information; it is like taking the blood pressure of your house. That's why Sheri Koones is right in saying, "Anyone thinking of buying a house or building one should consider having a blower door test performed." She writes in Forbes:
Results of the test can determine if there are unsealed cracks and openings in the house’s shell that should be sealed. Infrared cameras are used to locate every leak which can then be amended. Properly sealing a house will increase comfort, reduce energy costs, and improve indoor air quality.
A blower door test is one of the tools used to come up with a HERS rating, which compares the energy performance of a house to an imaginary "standard" house with the same dimensions and climate. Like the fuel efficiency ratings on cars, your own energy consumption may vary. HERS ratings are required for every house sale in California, and should be done for every house purchase, just as people demand home inspection reports; you should know what you are getting into.
It's certainly not perfect; as one energy consultant noted in Green Building Advisor, “It has always rubbed me the wrong way that you could have a 10,000-square-foot ‘energy efficient’ home,” because it simply would compare it to a 10,000 square foot imaginary reference home. But it is a start.
Blower door tests measure the air changes per hour (ACH) at 50 pascals of air pressure. A typical house might test at 8 to 10 ACH; a "pretty good house" should hit 1, and a Passivhaus can't exceed 0.6 ACH. New York State requires that all houses hit 3.0 ACH 50; we quoted 475 High performance Building Supply when that code change came in to force:
…even at “just” 3.0ACH50, increasing the airtightness of the building envelope will make a huge difference for the comfort and the energy efficiency of projects built to current code. These minimum code requirements are there to not only warrant the safety of the construction, but also to lower energy usage that will help achieve climate protection goals – while making buildings more comfortable and resilient.
All of the air that leaks in and out of the house has to be heated or cooled, and if you can control where the air gets in, you can filter it and extract the heat from it with a heat exchanger. It is really such an important part of knowing how well a house works, that I am surprised it isn't required everywhere.
As for my house, they went after every leak with a caulking gun; I got storms for my 100 year old windows, and it is not nearly as drafty as it used to be. I suspect they might even be able to get to 50 Pascals and actually test it now.