Since I started having problems with Nodejs and npm re El Capitan, I decided to find out exactly why one would want to install locally vs globally. By default, El Capitan still permits us to install npms locally, but not globally. I came across a video post regarding why one would install npm packages globally instead of locally, and decided to share it here.
How to modify System Integrity Protection in El Capitan: (In case you have software hindered by it) Glenn Fleishman,September 30, 2015, MacWorld
El Capitan’s System Integrity Protection will shift utilities’ functions: Glenn Fleishman, July 15, 2015, MacWorld
*Fixing npm permissions (I personally went with option 2 and skipped steps 2-4)
Fixing npm On Mac OS X for Homebrew Users (solution not recommended)
*Denotes official npm documentation.