There are four properties of the earth's surface which it may on occasion be important to preserve. One is area. On a world map, for example, one may wish to preserve the relative area of different countries so that a distribution of some phenomenon may be shown.
Another property is shape. In a classroom it may be imortant for a wallmap to preserve the shapes of states or countries so that students can learn to recognize and identify them.
For navigation purposes the most important properties will be direction and distance.
No projection is able to accurately preserve all four of these properties: area, shape, direction, and distance. There is always going to be distortion.
Some map projections are "interrupted"; they place the worst distortion over ocean areas and depict land areas fairly accurately.
