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.
Gazetteers are indispensable tools for placing your ancestor in the right place at the right time.
Geographical and political boundaries change over time, towns disappear or the name gets changed. Using a map contemporary to the time or event of your ancestor's life is helpful in understanding the physical challenges they may have faced.