The manifest stuff is so you can have a more declarative way to specify packages (think Cargo.toml instead of mutable Debian installation).

    (env)$ exit
    $ guix package --export-manifest -p ~/profiles/developer
    (specifications->manifest
      (list "guix"
            "coreutils"
            "emacs"
            "gfortran-toolchain"
            "gcc-toolchain"))
    $ guix package --export-manifest -p ~/profiles/developer > yourproject/manifest.scm
    $ guix shell -C -m yourproject/manifest.scm
    (env)$

Works the same as before. Now more declarative. And you can throw away ~/profiles/developer* now, it will be rebuilt each time (with caching, don't worry).