IME the garlic additives are no more than appetite enhancers. The best form of garlic is freshly pressed garlic. I have used NLS Thera+A, which has enough garlic to kill some parasites. I have seen it function as a parasiticide in this form.
Although it is debated, there is evidence that allicin, a chemical in garlic, can actively kill parasites (among other things). There have been scientific studies showing that parasites died when exposed to allicin. It is not just an appetite stimulant or immune system booster. Not to mention I have never seen anything explaining how garlic would even function as an immune booster.