You can just describe the effect you want for your system. Most common cases are done for you in NixOS. Like configuration of packages, systemd services, etc. But you could write your own. I submitted a pull request for a service, and then made a half-assed fork of a GUI for the NUR.
Could I just used a different distro and just installed it? Sure, but now people use my package
I could have figured out how to set the iptables myself instead of using some software, but we’re sharing solutions here so the next person can just write the package name and just use it.
You’re creating your own ad-hoc solutions with different benefits and drawbacks.
Portmaster wants to download its own updates. They could just go the Firefox “managed by your organization” route if they wanted to