You might not have noticed how many shows and movies have disappeared from streaming services over the last few years, but many people have.
At the end of 2016, Netflix has 4335 movies, in May 2021 they had 3622. If what you wanted to watch was one of those 713 movies, you might have been inspired to think, hey, maybe I don't want to rely on byzantine licensing agreements.
In addition, while several people are fixated on transcoding on this page, for reasons unclear to me, one hugely popular feature of Plex is sharing libraries with others, so you can grant your parents access to your library. It's not necessarily just "you" watching movies and show over again.
True story: Friday night I was at happy hour with former coworkers, and I brought up the movie _Attack the Block_. Turns out it's not available on any streaming service. Not a single one! You know where it is available? My Plex sever.
And that movie is worth watching again.