Once you have a number, you know how much room there is for improvement. If the house comes in at 19 ACH50, for example, you know there's a lot of room for improvement. This is where the Blower Door becomes a great diagnostic tool. With the fan running, the analyst can walk around the house and either feel for leaks with their hand or use some type of smoke device that will show where air is leaking into the house. 
One tool that has become popular among raters is the Wizard Stick. It's sold as an educational device and uses nontoxic 'smoke.' With the Blower Door running, you can walk through the house holding the Wizard Stick near places that you think might be leaking and watch to see if the smoke is blown away.
The photo here shows the Wizard Stick being held in front of an air conditioning vent during the Blower Door test. Notice that the smoke is blowing away from the vent, which means air is being sucked into the house through leaks in the ducts. In other words, leaks in the ducts hurt you twice. When the system is running, you have duct leakage. When the system is off, it adds to the home's other infiltration.
Blower Door Operation
Air Leakage at Elevated Pressure
Air Leakage at Natural Pressures