Earlier quoted context omitted.
I have not used flakes at all for my NixOS server. To put it one way, why should I care?
(speaking as someone who hasn't actually fully converted over to flakes yet) Because it actually fulfills the original promise of Nix : Forever-reproducible builds. Anyone who's ever dealt with a lockfile across any stack that supports that idea in dependency definition, knows how this works and what it's good for.
Using Nix without Flakes is going through 99% of the trouble for concrete, reproducible builds but then dropping the last 1%.
I had my non-Flake builds break like it was a mutable operating system. I just don't get why Flakes aren't the standard.