Quoting myself the last time this question was asked (https://news.ycombinator.com/item?id=8917505):

On top of its transport layer abstraction, socket.io adds various useful features including events, rooms, namespacing, efficient/convenient (de)serialization of both JSON and binary data, and over-the-network callbacks. I've been using it in websockets-only mode for various projects (you can limit the transports used), because I don't care for slower / less reliable transports but still want the extra features.

EDIT: If you'll allow the plug, an MMO trivia game show I made using socket.io (and three.js): http://masterofthegrid.net/