It makes sense.
Ive found the built in digital thermostats a bit variable. I have one Goldair oil heater rather accurate and switches off at sensible temperatures. Then we have another (two) with built in digital thermostats/timers and they are a bit rubbish.
If I saw another of the Goldair I'd but it immediately - however doesn't seem like they make them anymore :(
These plug-in thermostats seem to alleviate those problems :)
BTW had a look on ebay and ~$50 seems to be the going price for these type of things for the UK etc