Usually this is based on the purpose of the map. A map showing all the roads in a local city or jurisdiction is made for the purpose of people in that area being able to find where they are going and know where things are. A more general road atlas with just major roads and highways is intended for helping you know the basic geography of an area, help travelers get through the area, and emphasize the direction you are taking. A topographical map (which shows elevation) is good for people doing search and rescue missions, pilots who are planning to fly at a certain altitude, or even geologists who may be interested in studying the effects of erosion in a certain area. Distorted maps which show the whole world give you and idea of where things are in the world but the scale is off because it would be impossible to be detailed there.