>>nobody can see any use in a thermometer, except for medical purposes (to measure body temperature),<<

Well, no...

Even the cheap thermometers are somewhat useful for getting a general idea of how well your sleeping arrangements do in what kind of weather.

Minimum-registering thermometers are much nicer to have, but if you wake up cold in the middle of the night and have to add to your insulation or whatever, it's nice to be able to check the actual temperature. After you've gained some experience with how your sleeping bags, pads, tents and tarps do in different configurations, then weather forecasts for upcoming trips are much more meaningful and informative. Not that there aren't plenty of other variables, but it still helps.

Also, a little experimentation with a thermometer, especially on windless days and nights in the woods, will yield some surprising results about temperature differences that may teach you not to camp in those cozy-looking hollows and the bottoms of quiet valleys, where cold air is funneled in invisible rivers and settles in pools, and it can get a LOT colder at night than it is on the hillside just a little ways above...

So, I'd say that it's a useful tool. How tightly those concerns are related to immediate survival is another matter.