I'm no expert on it but I believe the optimal idea is for people with a decent hairline and very poor density to keep their hair at 0 guard and use the dots to make it look like hair. It rarely looks plausible but I have seen a few decent looking ones. Most of the images you'll see advertised are ridiculous completely sly bald, and super packed right across with a laser ruler straight line, and they look like a cartoon.
And they fade as well, actually needs to be touched up every 18 months to 2 years.