I've heard of immutable OS's like Fedora Silverblue. As far as I understand it, this means that "system files" are read-only, and that this is more secure.

What I struggle to understand is, what does that mean in practical terms? How does installing packages or configuring software work, if system files can't be changed?

Another thing I don't really understand is what the benefits as an end user? What kinds of things can I do (or can be done by malware or someone else) to my Arch system that couldn't be done on an immutable system? I get that there's a security benefit just in that malware can't change system files -- but that is achieved by proper permission management on traditional systems too.

And I understand the benefit of something declarative like NixOS or Guix, which are also immutable. But a lot of OS's seem to be immutable but not purely declarative. I'm struggling to understand why that's useful.