I've been told that it helps with their immmune system, and that they actually like garlic. I have no actual proof, however they haven't been sick since I had them, and whenever I see a torn fin, there were many cases where aquarium salt was never necessary (although it was quite minor as well). I don't know if I can say it DID all these things, however, I saw overall differences after I began the garlic, somewhere in those areas.
And plus, it has done absolutely no harm at all, and there has never been any trouble, except when I spilled and had to clean it up, haha.