EDIT: The only reason why I still had it at this point was because I could use it with other apps. However, now that my Spotify Subscription is cancelled, it doesn't work with anything. It's mildly infuriating because today, I can't still use it with other apps like I was able to yesterday.
Please don't make the same mistake I made. No one should buy this.
You can still use it to network with other computers over the telephone network. Heck you don’t even need to do that via the actual telephone service you can just run some wire.
But I think what was actually meant by OP is “tied to a specific subscription service” as well as “disables features that don’t need a subscription service when you aren’t subscribed”.
You can use them as e.g. smart home remote. The cellular modem is going to go unused (at least apart from emergency services) but that’s only a small portion of the hardware, and modems were only ever locked to subscriptions (at least over here) if the phone is subsidised by that subscription. I don’t think they even do that any more, they replaced it with minimum contract durations. In any case even back in the days you could unlock it after some time or coughing up some money.
Yep I wish rootability was included in the new EU regulations, it would solve so many issues at once. OTOH: It would solve the issue for people who are tech savvy enough to do such things, gotta be careful with our own elitism there. Enjoying consumer rights should not hinge on being a grease monkey.