I think what the parent is alluding to is: What is the practical point of using Nix when similar outcomes can be reached with more mainstream solutions? In other words, suppose I want to deploy a web app that depends on Nginx, PHP, and PostgreSQL.
I could:
(a) Run all as docker-compose type solution
(b) Run all as scripted Nix solution
(c) Install packages manually on general purpose OS
Lets ignore "c" for the moment as the goal is to avoid that exact scenario. Given the prevalence and widespread support for "a" why should sysadmins choose "b"?