[0] Marie Kondo would disagree
By saving stuff, you future proof the high possiblity that it becomes completely unavailable anywhere online and you have the last copy. And you can easily find it by search, I have this happen all the time. In many cases the audio cassettes that accompany an old pedagogic text have been digitized by one person who shares a zip one time and then the link dies forever. Ten years later when I finally free up time to study the material, it will be too late.
The important skill: don't download or even browse crap you certainly won't use within your lifetime. It is a time sink. Develop a strong preference for only looking for things you'll immediately begin studying or have a concrete plan to study in the coming 6mos.
It isn't always like that. If your data is on somebody else's platform (as for the author's data) your data is gone, possibly for good.
And by the way... onljne communities (and websites in general) come and go. The data you're relying upon today might not be there tomorrow (or it could get altered someway).
For some things yes, for TV shows and movies eh. For roughly the price of a year of netflix you could get a 14TB hard drive, which would hold six continuous months of high quality 1080p video. It's also really easy to keep those things sorted with negligible clutter and wasted time.
As much as this is technically true, anything not currently on your hardware can, and probably will, get snatched away from you, usually without warning, regardless of it you paid for it.