As stated in NPM documentation of main field

The main field is a module ID that is the primary entry point to your program. That is, if your package is named foo, and a user installs it, and then does require("foo"), then your main module's exports object will be returned.

This would work well when adding browserify support

Also many modules are starting to expose their css entry files in their package.json files via the style field . This allows tools like npm-css, rework-npm, and npm-less to import angular-time-picker CSS from the node_modules directory. Also this plays well with browserify plugins (like parcelify) which helps in bundling CSS together from npm packages