If you get stuck in traffic on the way home from work, you might have your heating on for an extra 30+ minutes unnecessarily.

If you decide to have a lazy day and sleep in, you can turn it on when you wake up instead of potentially 2+ hours early.

If you have an unexpected overnight stop over somewhere (e.g. after a night out drinking), you can turn it off until when you're home again. Also works if you're taken to hospital.

There are many valid uses for it.

The people in that article you posted specifically signed up to an 'energy savers' scheme that explicitly allowed the energy companies to change their smart thermostat temperature. They agreed to it. There's no reason to think that energy companies would normally be able to take over your thermostat.