Sure, if you have a program that outputs a random number, NixOS doesn’t make it suddenly always output the same number (would one even want that?). Nix aims to

Sure, if you have a program that outputs a random number, NixOS doesn’t make it suddenly always output the same number (would one even want that?). Nix aims to remove side-effects from the build system (and some aspects of the execution environment), not from within the program itself. Whether the program itself is pure is up to it, not NixOS. But the point is that NixOS eliminates whole classes of why the output may be different.