Lots of people use Plex etc for movie playback. Kids love to rewatch movies, as do some movie lovers. I have a copy of many movies in a disk array so that I can watch them anywhere in the house without digging out a disk.
I do store movies, and quite a few people around me too. I store only truly remarkable and important ones to me that I might watch later (and often I do), currently filling most of my 3 TB drive. I never go back to series, even the best ones are one-time watch for me.
Flac music can fill another 500 GB easily, more depending what you listen to..
Not a lot of people know how/can stream a properly good 1080p stream with high bitrates. oh, you want some subtitles because you're not an english native? Or you have fancy stereo/surround setup that you want to actually use? oh well, you better download some bigger version.