

To answer your question, most people don’t have just one device. Do you have only one device? You must have at least a desktop computer and a smartphone? What if you want to have something stored in your computer when you are not at home?
Music for example. If I don’t want to pay Spotify or whatever, and I want to listen to my music on my phone at work and on my computer at home. Other than making two full copies of the entire music library, I think I have to store them on a 3rd location then share it to my two devices.
If I don’t listen to music at home, then you’re right, there’s no reason to self host anything. I can just store all songs on my phone.
You could ask the question for video gaming. Can a used computer do the job? Yes, but you may not be able to play cutting edge / demanding games if your computer lacks the appropriate hardware. It really depends what kind of things you want to do, for choosing hardware that’s powerful enough.
Jellyfin? You need to consider if you need transcoding. Transcode or not makes quite a difference on the hardware needs.