Beyond knowing the name of the city or township where your ancestor lived, a good working knowledge of each location can be critical when it comes to knowing what records are available, where you can find them, why your ancestors made the choices they did. Mountains, lakes, rivers, and other natural hazards may have determined where your ancestor went to shop, worship, and take care of business. Maps and atlases give us a look at the lay of the land. Detailed maps may include other features like churches, cemeteries, government buildings, and in some cases even the names of property holders.
Gazetteers may include maps, but are primarily dictionaries of place names and contain descriptions of geographic features, transportation routes, and the economy of the area. They can also include historical information on seats of government, public institutions (e.g., prisons, schools, etc.), churches, and charitable institutions. Population statistics can give you a feel for the growth of an area and nativity.
Throughout history, boundaries and borders have been redrawn. Knowing where those lines fell during the life of your ancestor is an important part of family history.
Maps, atlases, and gazetteers can help us become familiar with the places where our ancestors lived, learn more about the environment they lived in, and what civil jurisdiction may hold the records they left behind.