Let's look at a typical suburban house in winter, let's call the house itself a large box.
The outside temperature is too cold to be comfortable for humans so we need to somehow heat this box we live in.
So, inside the large cold box (our house), we have another much smaller box and this is our heater and this small box is heating the air inside the large box.
Now, we also want to keep food in our large box and this food needs to be kept cold otherwise it will quickly go off and we will not be able to consume it.
So, how can we keep the food cold and separate from the air we have just heated in our large box?
That's right! we need another box but this one needs to be about as cold as the outside air temperature.
Can you see anything wrong with this picture? (apart from the fact that it was badly drawn).
Would it make more sense to have the cold box outside?