Just like today's text messages, typewriters lacked the ability to emphasize words, leaving the user to resort to using all-caps to make certain important words stand out. However, using caps for an entire message actually decreases legibility. The brain deciphers words based on their overall shape and the shape of the letters, including their acenders and descenders which capital letters lack.
The sample below is from the National Weather Service in the US. For years their forecasts had been posted in all-caps due to old equipment. Just recently they made the switch to using upper and lower case. It is quickly apparent how much easier the second block of text is to read than the first.
Resources: www.noaa.gov/national-weather-service-will-stop-using-all-caps-its-forecasts , https://www.fonts.com/content/learning/fyti/situational-typography/all-caps