The active component in garlic that is responsible for many of its properties (allicin) is unstable when exposed to air, it should be used within three hours of being chopped. This is not easy. I keep minced garlic in my refrigerator and use it at times. But when I need to use garlic for a specific health condition, I peel and chop it.
So why should we use garlic powder if the active component is just active for 3 hours?
While, the trouble with garlic is that it starts growing just after it is harvested. Right now, my garlic has a green sprout in each clove as I use them. Storage of garlic requires a space which is chilled and humidity controlled. That is how commercial produce dealers stores them.
The granulated powder is just another way to have garlic flavor well after the clove would have totted away. Fresh garlic is the best for the organics you speak of BUT reasonable flavor can be achieved by using the granulated variety.