It is basically a database, reporting tool, IDE and half a dozen other things all rolled into one giant mess.
They had looked at getting a proper purpose-built system in place, but I don't know what happened after that.
First: in classic systems the OS is a single application/framework where anything or nearly anything is accessible by end users, so there is no special integration limit that force devs to reinvent the wheel to have anything in a single modern app and reinvent it in bad way since they have no time nor resources even at the big tech sizes, plus the need to invent ways to circumvent system design limits.
Being a single app means if I have a CAS installed I can solve an ODE in an email just typing it and pass the math expression to a relevant CAS function, no need to reinvent a basic or less basic calculator. No need to know all the stuff about making it. I just access the relevant functions written by some expert and tuned for that purpose. The dev does not need to know, it's the user who know and use.
Secondly means an incredible simplification. Let's talk for instance about a Plan9 mail client: what kind of beast is? Well it's just a base64-to-text reader and writer. Nothing more. The connection to the webserver is just a universal system connection to a remote file server, a remote filesystem mounted somewhere under the local root. All you need to do is knowing how to access a local fs, read text files, read/encode base64. Nothing more. Sending an email is just saving a text file with a given name in the right place. Publish a website is the same BTW, so it's sharing a file. Being a single app-framework means that anything is simpler, there is not much duplication of functions, only tuning.
Now try to see an example of modern Emacs in action like https://youtu.be/B6jfrrwR10k how much complicated is creating a slide? Well just zoom some org-mode text. A table like a spreadsheet? It's just text and can be passed to any programming language supported by org-babel as data.
Doing so meaning no lock-in possible and the end-user in control, that's why all the modern IT industry starting back then with the not-so-modern IBM, have demolished this, but the result is a mess. A decade after another we tend to the old model wasting gazillions of resources to keep up an untenable business model.
From a business standpoint it's probably the single most useful software platform ever invented.