Courier
Howard 'Bud' Kettler created the font, Courier, for IBM's typewriters in 1955. This highly identifiable typeface is considered a monospaced slab serif because it mimics and the strike of a typewriter, and it leaves enough room to fix errors. This geometric feature of a consistent weight makes the font incredibly easy to read, which is perfect for important documents, HTML codes, and, of course, typewriters.
Courier's usage is so universal because of its accessibility. The type is used for such general things like ASCII and HTML codes because most machines have it available. At the time when typewriters became of common use and were introduced was when screenplay format became important, which is why it is still the font used in the field today. It is said that Courier somehow mimics exactly what the actors are intended to do, so it is perfect for manuscripts.
As for type designers, Courier is used as inspiration because of its "nostalgic analogue qualities," according to Font. The Sourcebook. Examples include James Goggin's geometric sans serif, Courier Sans, and Erik van Blokland's Trixie that looks like a dirty typewriter.
Today, Courier is associated to the font of a typewriter; however, in the 1950s, 60s, and 70s, each brand of typewriter had different fonts that were all recognizable to the brand. Many advertisements use courier new when the subject has to do with messages, emails, letters, and the like.
The name "Courier" is not what Kettler originally intended it to be called. He originally named the type "Messenger" and for unknown reasons switched to Courier. The word messenger is rather broad in a sense that it could be applied to many different kinds of deliverers. The word courier, however, is specific to a messenger of important documents. The name Courier for the font that was to be used in typing out messages was completely apropos, especially since it was used for American governmental documents.