The scents are not magic and are in part salesmanship but there is a time and place for them IMO. Fish do have and use smell/taste in their search for food just as they use sight and sound. Different species use these senses in combination and to different degrees based on their method of feeding.

Maybe we should ask why live bait works so well. Many lures look and move just like the real thing but fish don't bite them any better. Why? Part of the reason might be they don't sound , smell or taste right. Many of the scents are made of fish oils or craw oils and as such may in the right circumstance make the fish more likely to bite and hold longer. If you have the chance to watch the tape "Big Mouth" you should. It is the result of thousands of hours of underwater filming with a lot being watching fish eat (lures included). You would be surprised how many times fish bite and are not detected and how fast they can bite and spit out a lure ,even a large crankbait with 2 large trebles. Most of the time the pro fisherman on the other end of the line never knew either.

IMO fish smell a lot more than most give them credit for. If you can get the scent right then you may get a few more bites at the margin in the right situation if you can detect them and react in time. ;\)