Compare data used downloading vs streaming.
Streaming transfers the whole title every time you play it; downloading transfers it once. Whether downloading saves data therefore depends entirely on repeat viewing, and on the fact that offline downloads are usually encoded slightly lower than the peak streaming bitrate. Below the break-even watch count streaming is cheaper; above it downloading is, and the gap widens with every replay.
Repeat-viewing data comparison
Stream GB per watch = stream Mbps x 3600 / (8 x 1024) x hours; download GB = download Mbps equivalent x hours; saved = stream per watch x watches - download GB
Because most services encode offline downloads at a fixed moderate quality rather than the adaptive peak used for streaming. Standard-quality downloads are commonly 20-40 percent smaller than the highest streaming tier.
Usually, yes. When download and stream bitrates are similar, downloading pays off from the second viewing onward — and it pays off on the first viewing if the download quality is lower than the stream would have been.
Yes. Downloading over wifi and watching on cellular shifts the traffic entirely off the mobile plan, which is a bigger saving than the byte comparison alone. It also removes buffering on unreliable connections.