Even though I am usually a big advocate of using eyeball-based management and avoiding artificial heat unless frostbite is imminent or occurring, I am going to play devils advocate here because I think there *is* some legitimate use to knowing the temp in your coop.
(Mind, you can acquire almost-as-good information from just a mechanical max/min thermometer, which is all I use myself; but thermometers with remote displays have become so common and cheap these days that they are certainly an option too).
If a coop just "were", if it just sat there and was unchangeable and its physical characteristics unalterable, there would indeed be not much point in knowing the temperature inside it. (Even in terms of whether to add heat, knowing temperature doesn't tell you anything, because it is the chickens' health and behavior that is the deciding factor, not some digital-readout numbers). I mean, it's not like you could do anything about it anyhow.
However coops are generally MANAGED. You open or close vents on different sides, you maybe change how windows are rigged or whatever. In ways that change its temperature.
It is really useful, IMHO, to develop a reality-based sense of how your management actions affect coop temperatures. What difference does it make which vents are open, which are closed, how MUCH open or closed, in different wind conditions, how much does the coop warm up on a sunny day (if it's "a lot" you have the opportunity to put in barrels of water or something to trap some of that heat for nighttime...), etcetera.
So while I certainly don't think it's *necessary*, I do still think it's *useful* for people to get all data-collect-y about their coops as long as they have the enthusiasm for it. The information really CAN come in handy.
Furthermore, as far as "chickens can take subzero temperatures so I really don't care how cold their coop gets", well, SOME chickens can SOMETIMES take subzero temperatures. (For "some", read "many" if you like, I am not trying to start semantic arguments here). But it is not all *that* uncommon to get frostbite starting in milder temps e.g. because of humidity problems be they management-related or weather-related, or to have chickens struggling with the cold if they are a non cold hardy breed or suffering under the burden of some other health problem already.
So unless one is going to take a hard-nosed Darwinian "if they die, oh well, only the hardiest get to survive" attitude (which I'm not criticizing, it has some merit IMO but so do other attitudes too), you have to realize that sometimes you MAY find yourself wanting to add heat. And if you DO get to where your best roo is getting frostbit and you want to give him a bit of a lamp to minimize further pain and damage, or something like that, you will CERTAINLY benefit from knowing what temperature is like in the coop, because it helps you fine-tune your heating to use as little as necessary.
So, I dunno. Technology is not entirely useless, even if it isn't NECESSARY as such
JMHO,
Pat