A blower door is a significant diagnostic tool for measuring air infiltration rates and locating air flow pathways through and within a building's thermal envelope.  
This tool is used as an important part of energy efficiency certification programs, code compliance, and energy audits.

The blower door can simulate external pressures across the envelope equal to a 20 mph wind hitting all exterior walls simultaneously.  As the building is depressurized (or pressurized) a manometer measures in cubic feet per minute (cfm) the total amount of air that is being drawn (or expelled) from any unsealed areas of the thermal envelope.  The measured amount of air infiltration CFM can then be converted into Air Changes per Hour (ACH).

ACH is an important number in the overall efficiency of the building.  The lower the ACH, the tighter the thermal envelope is.  The tighter the thermal envelope (the lower the ACH), the lower the cost to heat and cool the building because the conditioned air that you pay for stays within the building longer.   And yes, a lot of money is wasted in the U.S. trying to heat and cool the outdoors due to envelope leakage.  The Department of Energy considers a residential building with 3.5 ACH or less to be considered a "tight" envelope.  This means that in an hour the building has exchanged inside air for outside air 3.5 times.  For older homes & buildings the ACH may be 15 or 18 ACH or higher.  

A blower door test can not only determine ACH but can also measure the area of leakage.  It is not unusual to find that some buildings have enough measured area of leakage that it would equate to leaving a door, window, or several windows open 356 days a year. 

Blower door tests also help a builder to determine whether proper air sealing practices were used by the subcontractors and particularly that the insulation was installed correctly and is doing its job.

Blower door testing is required by most energy efficiency programs (HERS & Energy Star) and the 2009/2012/2015/2018 and 2021 IECC Code. 

Asking what a building's ACH is can be an important question in the purchase process.  Particularly for a new building.  If the builder or Owner doesn't know then they cannot be sure of the performance of the building.