Are those different? If I chop up a video file into chunks, I'm streaming it, and if I save a stream I have a file. With a buffer, I would expect the sizes involved to be identical. (Although without a buffer I'd expect streaming to be worse)

Yes. For the most part, you need to be able to encode to a low and constant bandwidth at 30/60/+ fps, possibly even with limited latency. Then there are also some lesser, but also important aspects, such as the need to be able to start in the middle of a stream, handle lost packages etc.

Latency will become more important with video conferencing (already now) and AR/VR.

>Are those different?

No. In the end you transmitted a file that has a certain size. It doesn't really matter if you save that file or just use a volatile buffer.