The floppies got A and B because hard drives were expensive and for a time a lot of people got by without them, though everybody had at least one floppy.

Even if you only had one floppy, it was both A and B, so you could say "copy a:DOC.TXT b:" and it would read the doc and then ask you to insert the floppy you're considering B (and depending on file size and available memory, sometimes switch back and forth a few more times)