Updates, dependencies, offline development, etc. Not really a node.js thing (though bower itself can be installed via npm), it's specifically targeting front-end development.

The debate over CDN vs. local is debated on all levels. Usually CDN is faster due to cached files, but what I consider the "best" approach is to use a CDN for most everything with local fallback (yes, you can use bower packages for the fallback if you please).

Check the below linked poll out... http://css-tricks.com/poll-results-cdn-or-local/