Exactly what @Respectfully_Declined described. When making garlic bread I’ll add an ample amount of fresh garlic to the butter and Parmesan mixture, but will also add granulated garlic or garlic powder too, to evenly distribute through the mixture, so you’re getting the sharp and pungent fresh garlic, with the background flavor of garlic also. I’ll do the same for soups and stews, it’s a good booster. Also, it can be a short cut for some cooks who don’t want to mince fresh garlic, but I don’t typically use it that way. Oh, and another thing, it’s also good for grilling meat if you want a garlic flavor on meat or vegetables, but don’t want the fresh garlic bits to burn.
Almost forgot that granulated or garlic powder is pretty essential in dry rubs, and a lot of seasoning mixes if you make your own. An indispensable ingredient for lots of things. I use it in house made vinaigrettes in addition to fresh as well.