I guess you could consider the wooden frame that the abacus came in as the first computer case, but today we're gonna start by talking about the case from the original 1981 IBM PC.
The original Personal Computer from Big Blue was obviously a tremendously important computing platform, but the chassis was also notable for influencing what most PCs would look like for the next couple of decades. It was that standard beige box with a couple of floppy drives accessible on the front as well as expansion slots and a keyboard port on the back. And if you're wondering why it was beige, well, the answer is that nobody knows for sure, but computer historians seem to think it had to do with businesses that bought PCs wanting them to fit in with other office equipment at the time, which was... Well, you guessed it: mostly beige.
Anyhoo, PC cases didn't change too much outwardly for about 20 years, aside from the addition of 3.5" floppy, and later, optical drives, but there were some major changes on the inside when it came to form factor: the standardized size for key components like the motherboard and power supply.
A form factor called AT was popular back when the 286 and 386 processors ruled the world, but unlike today's cases, many connectors had to be built into the case itself, kind of like the front panel I/O that you see on cases today. Not to mention that the motherboards were larger.
ATX came onto the scene in 1995 and not only did it make for smaller PCs, but it introduced the modern concept of the I/O panel on the back of the motherboard.
Because ATX didn't rely so much on connectors that were already built into the case, it gave motherboard manufacturers much more flexibility as to which interfaces they wanted to support and made it easier for users to upgrade their computer without changing out the case.
ATX also introduced the familiar ATX power supply, which was really important because it allowed for software control of that front power button, so you could just press it to tell the PC to shut down properly instead of just killing the power.
But it wasn't until the late '90s and early 2000s that there was a big push from the industry to move away from beige boxes and really rethink both how people would tinker with their PCs and how computers would look aesthetically.
Now, you're probably familiar with how black PCs started becoming more common around this time, including the next computer that Steve Jobs was pushing before he returned to Apple. Also, how mass-produced machines from companies like Compaq started adding splashes of colored plastic. But one more interesting early trend was to give computers a more industrial look with a shiny metal exterior, as we saw with the 2003 Antec Lanboy. It featured a lightweight aluminum chassis to allow it to be taken to LAN parties more easily and even had a side panel window, so you could show off your totally sick Athlon XP and be the envy of your fellow nerds.
It wasn't just custom-builds by gamers that featured this aesthetic. Apple also jumped on this train with the Power Mac G5, with its famous cheese grater front panel that inspired the Mac Pros that you can buy today. Of course, not all gamers wanted that look, as this was also around the time you started seeing those all-acrylic cases that people would often fill with neon parts. Those builds did look extremely messy, but hey, they were a heck of a lot more interesting than buying a beige box from Dell.
Not long after, in the mid-2000s, 3.5" drive bays started to disappear, and when a case did still have them, users would often install a different device, like a card reader or a fan controller, to make use of the space because people simply didn't need floppy drives much once USB drives became a thing.
So people were now thinking, then, about removing unnecessary features from cases rather than adding more. Combine this with the continued popularity of windowed cases and design philosophies shifted towards making builds look more tidy and elegant, hence built-in cable management features becoming popular in the late 2000s, tempered glass in the mid-2010s, and of course more minimalistic aesthetics overall, driven in no small part by Fractal Design.
Even tackier designs like the infamous Antec Lanboy Air, which could fit over two dozen fans and features bungee supports for hard drives, used modularity as their ethos, so the case was supposed to be easy to rearrange or strip down as the user saw fit. It wasn't.
This was also a time where components were becoming more powerful than ever and real thought needed to be put into ensuring that systems were cooled properly. The Antec P180 from 2006... Those guys really weren't afraid to innovate, were they? Featured a separate chamber for power supply cooling, similar to today's power supply basements. And in the early 2010s, we started to see cases make excellent use of the space once taken up by clunky bays for optical drives and full-size hard disks and instead dedicate it to water cooling radiators and reservoirs. And even gamers not running a fancy liquid cooling setup often could use that space as one gigantic air intake. Oh yeah, and somewhere in there, RGB happened. People really like RGB. That did merit a mention.