SSE suits live dashboards, notifications and streaming AI responses, where the browser mostly listens. It runs over plain HTTP, reconnects automatically and can resume from the last event it received, using an ID the browser sends back. OpenAI and Anthropic both use it for their standard text-streaming APIs.
The main limit is direction: the browser cannot send messages back on the same connection, so it uses a normal request for that. Proxies that buffer responses can also hold events back until the buffer fills, which makes a working stream look broken in production. For two-way traffic such as chat or shared editing, a WebSocket is usually the better fit.