Researchers frequently use them to find ancestors when the researcher does not know where they lived. You can sometimes identify a relative with no more prior information than a name. The indexes often help trace the movements of a family from state to state over many years. They are an easy way to locate specific places where your ancestor lived and to identify when they lived there. The broad nature of these indexes also makes them valuable for identifying previously unknown places to search for relatives.
Few records reveal as many details about individuals and families as do the federal censuses.
The census is often the best starting point for further genealogical research. If nothing else, census records are important sources for placing individuals in specific places at specific times. When you can't find any family, vital, or religious records, census records may be the only means to find any details of a person's life. A census record provides information including the age, sex, race, occupation, and birthplace of each person in a household. You can also find information about whether or not members of the household attended school, are literate, or married within the year.