To make realtime apps possible in every browser and mobile device.
It supports several transport protocols, choosing the best one for the specific browser.
If you were to implement your application with WebSocket, it would be limited to the modern browsers supporting that protocol. Because Socket.IO falls back on so many alternate protocols (WebSockets, Flash, XHR, and JSONP) it supports a wide range of web browsers, including Internet Explorer 5.5.
As the application author, you don't have to worry about the specific protocol Socket.IO uses in a given browser. Instead, you implement the business logic, and the library takes care of the details for you.
Socket.IO requires that a client library make its way into the browser. To make that easy, the Socket.IO server listens for a request on a given path, which by default is /socket.io/socket.io.js, responding by sending the client library to the browser.
The model Socket.IO provides you with is akin...