Socket.IO is a popular library that wraps WebSocket with auto-reconnect, room support, and HTTP long-poll fallback. Native WebSocket is the browser standard. Choosing depends on whether you need the extras and can afford the protocol incompatibility (Socket.IO is NOT standard WebSocket on the wire).
What Socket.IO adds
Auto-reconnect with exponential backoff. Per-namespace + per-room broadcast (server can address subsets of clients). HTTP long-poll fallback when WebSocket is blocked. Event-based API (socket.emit('event', data)) vs raw message strings. ACKs for delivery confirmation.
The protocol incompatibility
Socket.IO has its own handshake and framing on top of WebSocket. A vanilla WebSocket client cannot talk to a Socket.IO server. If your backend isn't a Socket.IO server, the client doesn't work. Lock-in.
When native is better
Backend is gRPC bidi or raw WebSocket. Multiple language clients (you don't want to port socket.io to Rust + iOS + Android). Performance-critical (smaller overhead per message).
When Socket.IO is better
Pure Node.js stack. Need rooms/namespaces (don't want to implement them). Corporate users with WebSocket-blocking firewalls (the long-poll fallback saves you). Quick prototyping (the API is friendly).
Modern alternative: native + libraries
Native WebSocket + a small reconnect library (Reconnecting-WebSocket) + your own room logic in 50 lines = same outcome without the lock-in. For greenfield, this is what most teams now choose.