Because of updates and convenience. You could always use bower or the CDN alternatively. But since npm is the way I use most of the stuff I develop, that's what I recommend. You don't have to use node to use npm, but as a package manager it resolves lots of issues even for front-end components.

npm because it's now the de facto package manager for javascript.

And why a package manager? For many reasons, the most useful ones are :

- ease of install

- you can declare dependencies for your plugin instead of having the end user go to X differents pages to download everything needed

- the most important one : you don't have to check all your packages for important updates like security ones