It is a practice related with the now (largely) extinct custom of storing personal correspondence in the context of personal letters. It merely indicates that the letter was delivered by hand rather than via post or courier. It is only for future reference when the envelope and its contents are inspected. It does not mean that the letter was actually written by hand - it may have been printed on a typewriter cover sheet and then handwritten by the sender. People wrote by hand on envelopes because mail delivery was by postal rider until 1879. Post riders carried the mail from town to town on their backs, usually between 7am and 9pm. They left as soon as they dropped off the first bag of mail in the morning and didn't stop till they got back to their starting point at night. They were often chased by bandits who wanted their job! Bandits also stole mail so everyone had to be on the lookout for them. In some towns there was no post office until 1914 when the US government started giving out typewriters to select officials. They sent clerks to type up all the letters coming in to these posts and these typed letters were called "white papers". Then in 1924 the white papers were canceled and instead letters were mailed directly from Washington to citizens. This process of mailing letters directly from Washington caused many problems for individuals who had written letters to each other during times when the post office was closed due to lack of money to operate.