Some NixOS packages provide debug symbols. However, these are not included in the system closure by default to save disk space. Enabling this option causes the debug symbols to appear in /run/current-system/sw/lib/debug/.build-id, where tools such as gdb can find them. If you need debug symbols for a package that doesn't provide them by default, you can enable them as follows:
nixpkgs.config.packageOverrides = pkgs: {
hello = pkgs.hello.overrideAttrs (oldAttrs: {
separateDebugInfo = true;
});
};