These are the recommended dojo build tools for using the arcgis-js-api using npm. You can read more about npm here.
You would use the npm package of the ArcGIS API for JavaScript if you wanted to create local builds of your application.
This can provide an improved debugging experience as you will know which module an error originates from.
If you want to build your application using Dojo, we recommend grunt-dojo and the included build.profile. The npm package of the ArcGIS API for JavaScript is simply a minified version of the API.
The demo Dojo application can be found here;