What is the origin of the idiom "keep someone posted" and when to use it?

The idiom "keep someone posted" has its roots in postal communication and has been in use since at least the 19th century. In the past, letters and updates were sent through the postal service, and people eagerly awaited news from distant friends and family. This expression is used to indicate the act of keeping someone informed or up-to-date about a specific matter or situation. It implies a commitment to providing ongoing information as the situation develops, ensuring that the person remains well-informed.