Only reason I can think of is to traverse firewalls that block outbound sockets but don't block HTTP+WebSocket.

That aside, it seems a bit ridiculous, given that we already have sockets, to bolt on a protocol that exists almost solely to work around security issues involved in a browser opening a socket.

I think it's the same reason you would want to use good old HTTP over raw sockets for a request/response API, even though with a native client you could just invent your own protocol.

If you want bidirectional communication to your app, it's much easier to make a server that uses the WebSocket Protocol (tornado, etc.) instead of rolling your own server and protocol.