While Websocket has good supports by browsers, it doesn't always handle switching between networks correctly, also, some networks disallow Websockets altogether

While Websocket has good supports by browsers, it doesn't always handle switching between networks correctly, also, some networks disallow Websockets altogether, or drop websocket connections over HTTP, but not over HTTPS. Socket.io (especially > 1.0) solves these issues with a layer by having smart reconnection mechanisms and starts by connecting over plain old HTTP longpolling, and upgrades the connection to use websockets if they're available and work.