There are many health benefits associated with garlic and many of those crossover even to processed garlic powder. In fact, because the powdered form is so concentrated with its water removed, it actually tests out as having more antioxidants (a higher ORAC) versus fresh garlic.
ORAC Source
USDA Database for the Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 - Prepared by Nutrient Data Laboratory, Beltsville Human Nutrition Research Center (BHNRC), Agricultural Research Service (ARS), U.S. Department of Agriculture (USDA) - May 2010