It's used to power dynamic interactions in front-end content. It's used to power high-concurrency processing systems on the server-side.
Last fall I had the opportunity to speak in Russia about the power of asynchronous data processing - using new APIs to power systems from graphics processing to encryption.
The Internet of Things
As more of our household devices become interconnected, there is an underlying drive to standardize the ways in which they're connected. This standardization was part of the impetus for developing Node.JS for the server.
Imagine: a microwave with internals scripted in the same language that powers Facebook.
This is both a frightening and an exciting development with technology. On the one hand, standardization - anyone will be able to script their coffee-maker, their refrigerator, or air conditioner using readily-available and understandable code.
On the other hand, it means security issues that currently plague the Internet will also affect our non-computing devices as well.