It gives the viewer an idea of what the map depicts. Every map should have North arrow to demonstrate the map's orientation. The question of what is north can be an issue on some maps. Most maps we compose are oriented to true north, even though compass readings in the field are angled to the magnetic pole. We use maps of different scales to answer different questions.
Scale is the relationship between a distance portrayed on a map and the same distance on the Earth.
To find out how far apart New Orleans, Louisiana and Madison, Wisconsin are, we do not get out separate maps of Wisconsin, Illinois, Missouri, Tennessee, Mississippi, and Louisiana and then lay them together like puzzle pieces on the floor to measure the distance. Instead, we get out a single map of the United States. We need separate, detailed maps of each city so we can see all of the streets and their names and figure out the best route from one part of the city to another. To understand scale there are a few things you need to learn. Map scales are unitless.