Why do i want a copy of something I am only going to watch once? And why should I keep a copy if I can just stream it again from the same or some different site for free in the future?

1. you’re downloading a copy no matter which way you do it, you’re just tossing the copy as you download it with the streaming method

2. aside from the wastefulness of downloading the exact same thing multiple times, the source might vanish leaving you with nowhere to stream it from again