1. The hardware you buy for these activities, has still residual value after 1, 2, 3 year. Unlike the streaming service you pay for.
2. Its cheap to upgrade / expand over time (if its not a all in one solution)
3. It opens a door to not just store movies/music/images, but as emulator, streaming service, or game streaming to one or multiple.
4. The content will not arbitrarily vanish.
5. Your bookmarks / last viewed / ... will not arbitrarily vanish. Do not get me started on this and how annoying it can be when a services removes content!
6. It serves not only as a device for "linux isos" or other gray zones but also as a legit backup of your own personal data.
7. Saves you from needing "cloud" storage or other cloud services.
8. Can be enhanced with programs that offer image conversion, pdf conversion etc, all private!
9. Run your own chat server for the family, no US/EU "we want to know what you are saying" issues.
10. Can act like your own VPN, to route data from your phone or other devices outside your home.
11. Provides service if you are in area's with horrible internet connection with its ability to "cache isos" at night slowly.
12. Your control over the media means you can stream 4k to your PC. Netflix kuch kuch ... No, its not 4k.
13. You can gain the FULL bitrate of the media. You do not get a washed down version of the supposed media based upon how busy a streaming service their servers are or other limitations.
14. It can be used for so many other activities like programming.
15. Did i mention home automatization?
And so much more ... People are probably doing things with NAS setups that i can not even think about.
Your not investing into a machine for "illegal" stuff, your investing into a machine that frees you as the end user from all those cloud, streaming, and other services their lackluster service. And then provides all the added benefits on top, that a 24/7 running PC can provide.
Lets also not forget the future where LLM's are a thing. Having your own open source LLM that runs at home, can be a major benefit.
But ... it does require more knowledge, especially as you step up beyond simple storage. So that is the real downside, not the money, the time and knowledge buildup.