The spread of these letters meant that people were putting in effort to spread them (presumably believing making copies would make them rich or help them avoid bad luck). To make copies, people had to manually write or type up their own copies of the letters (or later with photocopiers, find a machine and pay to make copies). Then they had to pay for envelopes and stamps to send it in the mail. As these letters spread we could consider what factors made some chain letters (and modified versions) spread more than others, and how the letters got modified as they spread.
This is actually really funny because this is exactly what was happening when I was in middle school. There were always pictures that had some sort of "repost this for good luck" or "share this with someone else or you'll get bad luck." I think this is still pretty prominent on platforms that mostly host older users because I still see my aunts and uncles reposting things like that on Facebook.