Because Nix is a functional package manager, and functional package managers are the future of package managing (whether nix, guix, or something we don't have right now). It makes it easier to deal with supply-chain, to deal with rollback on your software updates, to distribute the same configuration everywhere…
I found NixOS (the OS centered around Nix) having similarities, in the way you configure things, with how you configure stuff in the networking world (with options that do some stuff behind the scenes you don't really care about). The difference is that with NixOS, you can dig in if you want to.
Also, once you used NixOS for a little while, it's very hard to go back to, let's say, Debian, Ubuntu, or Archlinux : manually configuring things in multiple configuration files in /etc seems really primitive.
I found NixOS (the OS centered around Nix) having similarities, in the way you configure things, with how you configure stuff in the networking world (with options that do some stuff behind the scenes you don't really care about). The difference is that with NixOS, you can dig in if you want to.
Also, once you used NixOS for a little while, it's very hard to go back to, let's say, Debian, Ubuntu, or Archlinux : manually configuring things in multiple configuration files in /etc seems really primitive.