Fish have a powerful sense of smell that helps them home in on prey. Humans emit a chemical called L-serine that has been proven in scientific studies to repel fish. Other repellents include tobacco, gas and oil, sunscreen and bug spray. So the number one rule before using scent is to keep your hands and gear clean. A wee bit of dish soap and a good rinse will do the job.
While the scent of prey will attract a target fish species, studies have shown that the natural scents of garlic and anise will mask human odours. For artificial scents to work, they must be water soluble and persistent. As others have mentioned, this makes them messy to work with. However, there is no doubt that masking human odour and applying an attractant can up your chances of fishing success.