Use CDN if you are developing a website that will be accessible by public internet users.

CDN Benefits:

npm is a great tool for managing your app's dependencies using a module bundler.

Example:

assume using a webpack module bundler and jQuery is installed

import $ from 'jQuery'
...
var content = $('#id').html();

but the browser does not understand the import statement so you have to transpile the code with Webpack commands, the bundler will check all the used dependencies and bind them in a single file without any dependency problems.

Source: javascript - Using CDN vs Installing library by NPM - Stack Overflow