The flake has a defaultPackage attribute, so you can nix shell to put the packages it defines on your path.
This can happen if you don't declare a defaultPackage in the flake:
$ nix shell
error: flake 'git+file:///home/dsyer/dev/scratch/flakes-demo' does not provide attribute 'packages.x86_64-linux.default' or 'defaultPackage.x86_64-linux'
You can still use the flake to set up a shell with nix develop:
$ nix develop
$ hello | cowsay
_______________
< Hello, world! >
---------------
\ ^__^
\ (oo)\_______
(__)\ )\/\
||----w |
|| ||