Duct blast testing is similar to how a plumber tests water lines for leaks.
We use a fan to put pressure on the duct work and calculate the extent of leaks. In sum, supply duct leaks can cause the loss of conditioned air, which you have paid to heat or cool, to the garage, attic, or crawl space.
Leaks in the return ducts can cause indoor air quality issues as they pull air in from the crawlspace, attic or garage. But it also causes your equipment to work harder as it is adding hot or cold air, depending on the time of year, back to your air handler and then through the supplies.