1. This stems from JS having literally no stdlib or convenience functions to speak of.

2. Array.isArray was added to browsers in 2009-2011. The time when IE 6 was still alive and kicking. No idea when it was added to node

3. This package plugged a hole in the stdlib, so obviously people started using it

4. It does on thing, and does it well. Unix-way!

5. How is using it any different than having the same code in a 20000-line utils.js file that any project inevitably grows to have to plug holes in JS's libs?