Before the late modern era, described historically as starting from around 1750, literacy rates were very low, in the area of single digits as a percentage of the total population. A rather optimistic projection from Our World in Data places the literacy rate at 12.5% worldwide in 1820, although it is important to remember that this was severely different across the socio-economic classes.
From a military standpoint, this meant that the average footman was probably illiterate, as was his family back home. This allowed only three methods of communication. The first was to ask an intermediary to write out a letter and then maybe another intermediary to read it out to the family. The second one was to send word verbally, through a person traveling from the army to the home region, with the expectable reduction in fidelity and length of the original message. The final option, and probably the most common for most of human history, was no communication at all. The family and the soldier would find out about each other’s fate only when the soldier returned home. However, as is the nature of war, many soldiers never returned home, leaving their families in the dark about their eventual fates.
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The warring aristocracy, on the other hand, was probably far more literate. Handwritten letters were far more common both on the personal level, between family members, and on the official level, with military communications between different parts of the army or even with the enemy or neutral parties being done in handwritten form.
A very interesting letter from 1310 between Robert the Bruce (King of Scots 1306-1329) and the King of England, Edward II, has survived to this day transcribed into a manuscript from the 15th century. In it, the Scottish king is asking for peace and the recognition of the independence of the Scots, but to no avail. Nonetheless, this is a valuable historical source showing how diplomacy worked.
As time passed and literacy grew, battlefield letters became more common. By the time of the American Civil War, these became relatively common, so much so that Shapell.org currently holds around 60 different letters and documents, a good portion handwritten, from the era that are digitized and freely available. These range from letters of common soldiers to the correspondence of the President himself, and provide a welcome window into life at that time both in the high seats of power and the bivouacs of the army.
With the start of the First World War, with its massive armies and the growing rates of literacy, especially in Europe, where the heaviest fighting took place, letters to and from home became far more than a personal pleasure. The safe dispatch and arrival of these small pieces of handwritten mail became a morale-sustaining military imperative, with significant effort being dedicated into the work of the post of the many armies involved in the conflict. According to the British Postal Museum, the London Home Depot that handled all correspondence from and to the front had to deal with 2 billion letters and more than 100 million parcels over the 5 years of war. For 1917, this meant 19,000 mailbags transiting the English Channel every single day, bearing good news, updates from home, or condolences from the front. The British National Archives have thankfully digitized and freely published a large number of such letters.
The Second World War was more of the same but, with the United States’ entry into the war in 1941 and landings in North Africa in 1942, the sending of mail to the American soldiers brought new challenges. Not only would these have taken a long time to reach their intended addressees, but the enormous volume of the letters themselves would have occupied valuable space and tonnage on the cross-Atlantic voyage that could have been used for better for vital supplies sustaining the Army and its Allies.
In order to solve this conundrum, the Americans introduced V-Mail (or Victory Mail) in March 1942. This took the letters and transferred them to microfilm, basically scanning them. This allowed up to 1,500 letters to be compressed into a small roll of microfilm weighing just 140 grams (5 ounces). This not only made them far smaller and lighter, saving on shipping tonnage, but also meant that air mail was actually viable, drastically cutting down on delivery delays. Many such letters can, for example, be found freely in the digital collection of the State Historical Society of Missouri and other such organizations.
Nowadays, with the rapid proliferation of electronic communication, emails, phone calls, or video calls, the need for handwritten letters from the battlefield has nearly evaporated. Nonetheless, at least the US Army still maintains the Military Postal Service, providing free shipping of letters and parcels to and from servicemen.
Nonetheless, handwritten letters can still play an important morale-boosting role even today. Several American organizations work to send letters of thanks, appreciation, and gratitude to US servicemen deployed around the world, either digitally or in physical format. You can find more information about this on Veteran Aid.
Sources:
https://ourworldindata.org/literacy
https://blogs.bl.uk/digitisedmanuscripts/2013/06/robert-the-bruce-letter-found-at-british-library.html
https://www.shapell.org/collection/?collection=6&featured=6
https://www.postalmuseum.org/collections/ww1/#
https://www.nationalarchives.gov.uk/education/resources/letters-first-world-war-1915/
https://postalmuseum.si.edu/exhibition/victory-mail
https://www.dvidshub.net/news/281450/brief-history-100-years-us-military-mail-services
https://digital.shsmo.org/digital/collection/wwii/search