What is this good for?

[Baughn]

...in the best possible way. ;-)

The readme describes a little bit of how it can be used, but not how it's superior to invoking nix-shell directly. Since nix-shell is bundled with Nix, and this is not, I think you need to at least add a feature matrix.

[bennofs]

I think I created the project before nix-shell could be used as a shebang. I am not actively using it anymore, though one advantage it has is that the syntax is easier and that you can use nix-scripti.

[michaelglass]

a similar project is in active development here