Cesar,
Garlic has two purposes. In the ocean nothing smells like garlic (at least that I know of) and because of its strong odor the fish quickly associate food with its smell. The fish response it remarkable to witness. They have a 10,000 times more sensitive sense of smell than a human. Come by the store sometime and I'll show you a little test I do.
Another benefit is in parasite control. Garlic helps reduce the number of parasites that find a host. It is not a cure-all but is an aid. It is thought that like a person who eats a lot of garlic and then you can smell it on their skin, fish have a similar reaction. The result is that parasites have trouble locating their prey. It also irritates the parasite when it tries to attach.
Regards,
Kevin