In modern parlance, a highly customised computer, often built for a specific task.
"I wanted a portable that would let me do X"
— a paraphrase of every cyberdeck builder
For many, it's about the challenge of doing something unique that expresses something of themselves. Some have specific tasks in mind, and design accordingly. Others are all about the bling, some are the real-world equivalent of a Star Trek tricorder or Fallout's Pip Boy.
People come in all shapes and sizes. They have different abilities; some excel at sports, some are good puzzle-solvers, others artists, teachers or artisans. Then there are coders, writers, data hoarders and hackers of various colours. And in an ideal world, each would have a computer dedicated to their needs, abilities and wishes. Then there's a group called "makers", and these are the creative people who are able to twist raw materials to do their specific bidding.
When makers and others intersect in the arena that is personal computing, strange things can happen. All of us here (I'm assuming) have access to some sort of communicating computing device, and usually more than one. For my part, I've a mobile phone and two laptops. They are used mostly for, well, the things most people use them for. Web browsing, email, writing, consuming media, preparing documents, some light coding. The odd game. Whilst this is fine for me, for those makers with particular needs, this is either too much or not enough. So they set about building something that fits their needs precisely.
For some it's a machine that's deliberately limited, for others, it has abilities that few others ever need. A few examples will do to begin. Consider someone who wants to just be able to write. It needs a really good keyboard, doesn't need a fancy screen or internet access. It needs no software other than that required to write, it needs to be portable, comfortable to use and it needs a long battery life. This machine might have an low-powered processor or single-board computer like a Raspberry Pi, it might use an e-ink screen because it sips power, and possibly some means of transferring data to another machine (for printing or what-have-you later). It needs to be portable and because it's going to be thrown in a rucksack and used anywhere, it needs to be sturdy.
Another is a blue team (or red team!) hacker doing system penetration testing. This machine may need to be stealthy yet support many wifi and network devices. It might need to incorporate all the functions of a rubber ducky device or a Flipper Zero, with things like NFC reader and writer.
How about the needs of a radio ham? This machine needs to be able to suck in radio waves, process them in some way and output whatever radio hams use. This one has a software-defined radio built in, needs to be a substantial build because it will likely get use in the field. It may not need to have a battery because the operator is likely to have more than one source of power for his other kit (yes, I'm making that assumption as the vast majority of hams are men).
Then there are the data hoarders. They want a machine that will store and display information or media. Some hoard music, some films. I have a friend who hoards books and manuals. Then there are those who have even more specific needs; I've seen survivalists whose decks are designed to survive a "shit hits the fan" scenario, in which civilisation collapses. One build² I've seen is basically a portable Faraday cage containing a resilient Raspberry Pi system with a built-in network switch so he can supply information to other survivors. Then there's an ultimate prepper PC with an offline wikipedia, survival guides, many books and everything to help in surviving and thriving in a post-apolayptic world.
Finally there's the build quality. Some are expensively built with expensive support technology like 3d printers, laser cutters, CNC routers and the like. They are built from aluminium, titanium, plastics. Others are building with wood and handtools, LEGO™ and old technology. Some are cobbled together seemingly with workshop scraps, others are polished and look professional. Sometimes makers are reusing technology in a way that makes my soul happy. A classic example would be reuse of an old phone or a laptop motherboard (Framework's upgradable machines are going to make many future cyberdecks!)
For some of these devices, a standard operating system and software will suffice, for others there's a need for specialised software. The example I've looked at can be seen on here. Built around a Raspberry Pi, it uses custom software purely designed to write (and available for download, because it's of course open source¹.
Whilst most of them are a far cry from William Gibson's portrayal of a cyberdeck, these unique and fascinating machines have found a place in the heart of many a geek.
¹ WareWoolf at Github
Cyberdeck Competition
Sometimes it's just because one can
² A survivalist deck
Iron node 6
$ xclip -o | wc -w
801