SSR with React and Next.js helps improve the perceived loading performance by showing a non-interactive page to the user as soon as possible.
However, it can still be slow as all data fetching on server needs to be completed before the page can be shown to the user.
Streaming allows you to break down the page's HTML into smaller chunks and progressively send those chunks from the server to the client.
This enables parts of the page to be displayed sooner, without waiting for all the data to load before any UI can be rendered.
Streaming works well with React's component model because each component can be considered a chunk. Components that have higher priority (e.g. product information) or that don't rely on data can be sent first (e.g. layout), and React can start hydration earlier. Components that have lower priority (e.g. reviews, related products) can be sent in the same server request after their data has been fetched.