I can give you an answer from someone who regularly downloads really old EOL versions of Ubuntu and Debian. I personally use them as part of attack and defense competitions. They are normally very close to unusable and are nearly impossible to update to a more recent or secure version. This forces my team to find creative ways to keep them working while also taking measures to isolate them as much as possible. I also use them to teach old exploits that have been patched in more recent versions, walking people through how it worked and why it existed.
It happens a lot more with Windows machines, but there might be some manufacturing systems out there that require software that won’t run on modern versions of the OS. These systems often require new manufacturing tools in order to upgrade, or they need massive overhauls that smaller companies can’t always afford.