Why should this nix-env command without -b download pre-built packages?

Quick Start - Nix Reference Manual says:

Install some packages from the channel:

 nix-env --install --attr nixpkgs.hello

This should download pre-built packages; it should not build them
locally (if it does, something went wrong).

Why “This should download pre-built packages”?
Does that require -b, which is absent in the example?


By default nix tries to fetch the pre-built packages from http://cache.nixos.org/. If there are none available it will build them from source. This flag will prevent this and just not install the package when there is no binary pre-built.
I hope that made it a bit clearer, feel free to ask more if you need more information.