HTTP is email. WebSocket is a phone call.

HTTP is email. The client sends a message and waits for a reply. Each request is its own conversation. Websocket is a phone call. Both sides stay on the line. Either side can speak at any time. When the server wants to push something to the client, it just speaks. That is the whole difference.

Use websockets when the server has something to say that the client would otherwise have to poll for. Live stock updates. Chat. Collaborative editing. Live dashboards. Anything where the answer to 'what is the latest value?' changes faster than the client is willing to refresh.

You can, and for some use cases it is fine. Polling wastes server and client resources when nothing changed, and it introduces a latency floor equal to your poll interval. Websockets are pure event delivery with no polling cost. For one client, polling is cheap. For ten thousand, websockets are obviously the right tool.