The following is "as I understand it." I hope someone with deeper knowledge of the subject will correct errors.
Fiche books date from pre-digital times. It was about the only way to "scan," store, and copy printed comics. I'm hazy on the exact nature of the microfiche process. It was related to microfilm, a photographic process which copied only in black and white. Libraries kept archives of old newspapers on spools of 35mm film which were viewed on big tabletop machines that projected an image onto a ground glass screen. (Our local library still maintains a microfilm newspaper collection.) I remember that dealers (in the 70s and 80s, I believe) sold Golden Age fiche collections through the mail. What I never understood about color microfiche is why fiche copies are invariably out of focus. Seems to me if you're shooting on 35mm color film you'd get a better image.
The more I write the more I realize both how little I know about fiche comics, and also how important they were to comics history. As awful as fiche copies are, it's good they were made, because many rare and valuable comics are available only in that form. By now the original books have been slabbed and removed forever from public view.