Ember why do we have to use import for certain bowr dependencies

In an Ember app, when using certain dependencies like moment installed via bower, we have to also import the same in the ember-cli-build.js file:

app.import('bower_components/moment/moment.js');

My question is why is that needed, since I would assume everything inside node_modules as well as bower_components should be available for use inside the app. Also if that is not the case, how do we identify which modules would require such explicit import to be able to use them ?