What is the significance of the purloined letter?

According to G., a letter has been stolen from the royal apartments that the police know the thief will use for blackmail. The letter belongs to a lady who was forced to hastily place it on a table when the person from whom she wished to conceal the secret entered the room. The Minister D., who also entered, saw and interpreted the contents of the letter correctly. He then placed a letter of similar appearance beside it before retrieving the incorrect paper prior to leaving. You can read about it at the GradeSaver link below: