Back before States and counties required the filing of a birth certificate with the county, the info of births and deaths were recorded in the family bible and then handed down to the eldest family member to pass on the records of the previous generations. As in my Grandmother's case she needed to get the family bible from her oldest sister and had to take it to Social Security to use as proof of when and where she was born. I recall thinking what a weird thing to have to take the bible to some stranger to prove you were born someplace when I was a kid. Also, some are used as back up in event the court house where the birth was recorded burned down.