I might not be able to give you the best explanation but here it goes. Garlic works in fish as it does with human. It kindda helps you build your immune system. In fish the garlic helps the fish to protect themselves from parasites. I guess that is the simplest way of putting it.
About using it regularly, I don't really know, though I did try it once. My fish hates it. Somehow he can taste it, he puts it in his mouth and spits it out again. Garlic is meant to be immidiately used when you notice any kinds of weird behavior that might be caused by parasites, such as scratching against the rocks. Garlic is also known for curing mild cases of ich.
I know others can explain it a lot better, but that's how I know it. So I hope it helps a little, good luck.
stupid_naso