It is used to shape maki and other sushi rolls into even cylinders with just the right amount of pressure - firm enough to hold together, but not so tight that the rice is crushed.
A sushi mat can also be used for more than sushi. It can be used to help roll omelettes, rice paper or cake rolls, and is sometimes used as a coaster to give texture to drained ingredients. Simple, quiet and completely dependent on the user, that's what makes it such a Japanese tool.