Edit: not sure why I'm getting downvoted. Is it that offensive to question orthodoxy?
Groups are a bit more niche IMO, but without groups there is no real other way to express the constraint of thing X uses file A,B, thing Y uses file B,C - how can they share the data without making things globally accessible or duplicating it. That’s probably a less frequent occurrence, but does come up (but again more on servers than desktop).
I wouldn’t say file permissions are necessarily outdated. You can get pretty far using something like systemd capabilities, which are much more supplementary to the existing Unix file permission model.
The very best approach to this IMO is OpenBSD pledge and unveil. Managing the permissions as part of the binary itself is much less fragile, although it doesn’t have exactly the same threat model. It also has the added benefit of temporal safety, which I’m not sure you can do with SELinux.
And of course if one has a family then one might want accounts for Mom, Dad, Alice and Bob.
You can no longer trust 3rd party applications to stay in their lanes. Running an application with full access to everything that I as a user have access to seems insane in 2024. Ideally, I don't want a third party application to read or write anything outside of its "home directory" without my explicitly giving it permission. That includes files on my filesystem, network shares, hardware devices, everything.
That's how Android works.
Tangent: one of the most talented engineers I ever met gave an amazing (tho sadly company-private, unrecorded) talk about how the OS was his IDE. Kind of analogous... anyway, I like this type of re-framing or meta-level-shifting.
In addition to this, my wife has an account on the computer which is separate from mine.
Which, as others have pointed out, means various system services running as other users (since you don't want them running as your user, and you also don't want them running as root). On most desktop unix machines that only one person uses, that's the main use case for multiple users (and for multiple groups since groups are used to manage access to various functions like printing, usb sticks, cd-roms, etc.).
Groups are a bit less useful (IMO), but still good for handing out access to things like device files. If a daemon should have permissions to XYZ /dev file then you add them to the group associated with it.
The most obvious reason for this is so that a security problem in one of these daemons won't be able to read your Firefox cookies, install a rootkit, and stuff like that.
It's understandable too, because Linux doesn't provide a nice way to retrieve that information. You'll have to parse /etc/passwd (which might not contain all users) or hope that whatever language you're using supports path extrapolations so that you can do realpath(~username) or call a third party tool that does, like a shell.
As for permissions: on a single human machine, sudo is useful principally to introduce friction to random install scripts doing random things to my system. In day to day use, there's no reason for me to input my password just to install some Firefox snap or something.
I know this can be done in Linux using flat packs, snaps, and the like, but I would really appreciate if sandboxing could be done at a more fine grained level, without coupling sandboxing and distribution.
I do, and not just for system services as mentioned by others.
I have separate user accounts for general desktop use, gaming, software builds, software testing, and a variety of containers.
Isolation is useful.
Somehow it slipped in for phones and that’s a big part of why they suck. E.g. you can’t have work, life, private/second life and tmp/trash accounts on your phone and have to either carry multiple devices or mix lives together.
I find them valuable. For example, I have a workstation that is used for different projects with different clients, as well as administrative work for my own business. I want 100% separation between assets related to those different contexts.
It’s bad enough that we have package managers allowing package installation scripts to run arbitrary code, or software wanting you to install via:
curl https://example.com/imnotmalwareipromise.sh | sh
At least if I have separate users for each client or each major project then the worst that is going to be compromised by a vulnerability introduced during the work for that client or project is that same work.
It’s not just about security though. It’s also about convenience and manageability. Those different clients and projects frequently require the use of specific security credentials and configurations, often for remote services that other clients/projects also use. In a perfect world, I’d like all of the software I use to be XDG-friendly, and I’d like each client/project to have its own home directory with its own independent XDG-style directories underneath, so each user has the configurations and credentials required for its own work and has no knowledge of or access to those of any other user. Finished a project? Archive/nuke that entire user and home directory as appropriate, and nothing is left lying around to break anything or leak anywhere later.
I’m currently playing with NixOS, which means I can also have a limited set of system-wide software installed and have specific additional packages installed per-user or even activated on demand when I change into a specific directory. Again, this means my system has only the software I actually need available at any given time, at the exact version I need for that specific work, and if something is no longer needed by anything I’m doing then it will automatically get cleaned up next time I do an update/rebuild.
None of this really works without the concept of separate users running different software in their own isolated little worlds, possibly concurrently on the same workstation and even sharing the same input/output devices (in a safe way where again they can’t unreasonably interfere with each other – something else that is not 100% there yet, but certainly a lot better than on de facto single-human-user operating systems). The only real alternative is to spin up something like a different virtual machine for each client/project where everything from the OS down is isolated, but I don’t really gain anything by doing that and it’s potentially more work to set up and more difficult to share input/output devices.