In the dark ages, when printers were PostScript and more powerful (and expensive) than the computers which printed on them, you added fonts by installing additional hardware modules, similar to a game console cartridge.
The links someone else shared are pretty cool, a little before my time (born late 80s). That's kinda neat, it makes me think of a post on HN some time back by Donald Knuth on the letter S and the technical challenges back then about that as a letter, mathmatically. I'm guessing given that, that's why printers needed these modules and were set up this way. Neat!