The postcard is a common form of communication even in these modern times. They may contain a great deal of information. For instance, the name and address of the recipient will be on them, and there is usually a date stamp. The correspondence on the card might contain a wealth of information. In addition, names and relationships, locations, and stories about the family can all be of great interest to the researcher.
A post describing the development of the modern postcard may be found in this link about the Detroit Publishing Company. The New York State Library also has a large collection of postcards from donations.
By 1907 postcards featured divided backs, such as the one below. This allowed the sender to write a short message on the card along with the receiver’s address.