Comic Strips

Usually back then cases had retention slots to hold on to the cards. This was a hold over from back in the days when computers were just card cages. So if the card didn't need any connectors, it simply wouldn't have any backplate. Another reason for no backplate was smaller computers, more in the style of earlier home computers. They often had stuff like a disk controller put somewhere flat, to keep the size of the case down.

The early XT and AT times were wild with all sorts of weird and fun form factors. We've become so accustomed to standards these days. But that primarily due to IBM becoming really dominant so people would make software, hardware and accessories compatible with IBM to target the biggest market. That in turn lead to other companies copying IBM into so called clones which were mostly compatible (and a real pain in the butt when the compatibility wasn't quite there). For a long time the standards were just do whatever IBM does.

This in turn hurt IBM, because it limited what sort of things they could change. If they broke compatibility, the new product wouldn't sell as well. We've seen the same thing later with Intel attempting to get away from x86 and the market refusing. For years ISA would still be on the motherboard, even though slots for it and things like IDE had gone away. I think some super IO controllers these days still have some parts of ISA in them, although I might be wrong on that.