I just want to add jailbreaking iOS is still very much a thing today. I’ve been doing it for several years. You can take a look in r/jailbreak, as there is not a comparably-active community in Lemmy.
As for the main reason, as others have stated, is to have root-level control to your device, which alows you to install alternative stores, which besides providing apps not sanctioned by Apple, also provide tweaks to alter various things in your system (i.e. alter your homescreen look and feel, change your statusbar color and icons, notifications behaviour and style, etc etc.).
I would look into either a permissions issue (the daemon having no permissions to write in your target folder), or a faulty disk issue. I’ve had a similar issue in the past and the former was the cause.
Your description seems coherent with the torrent downloading normally until the memory buffer is full, and then failing when the data is attempted to be written to the disk.