Why would someone implement the chunk logic when websockets are here? Am I missing something? What are the use cases?
You can absolutely assume that http 1.1 will work on basically anything; websockets are more finicky even now, and certainly were back in the day.
Whaaaaa??? We should eliminate these non-JS filled nonsense immediately!
Websocket is a different protocol that is started up via HTTP.
For example, some websocket servers don't pass back errors to the client (AWS). That makes it quite difficult to, say, retry on the client side.
Chunked encoding is used by video players - so you can request X bytes of a video file. That means you don't have to download the whole file, and if the user closes the video you didn't waste bandwidth. There are likely more uses of it.
Just a nitpick, but what you describe here is byte range requests. They can be used with or without chunked encoding, which is a separate thing.