This seems to be begging the question of whether it's possible to have a well-tuned fitness function that doesn't map in some significant way to the underlying reality.
Sure, maybe there's no point in us knowing the difference between hot and cold, as long as we avoid contact with both of them. Only wait, there is a point - if we know the difference, we can take different actions in response to each in order to respond in a more survivable manner to both. And this is indeed what we see - as well as abstracted signals (like 'pain') we have signals which measure our environment more directly (like 'hot' and 'cold'). These signals raise our fitness because they let us know not only that we should take action, but in what direction that action should be taken.
Sure, maybe there's no point in us knowing the difference between hot and cold, as long as we avoid contact with both of them. Only wait, there is a point - if we know the difference, we can take different actions in response to each in order to respond in a more survivable manner to both. And this is indeed what we see - as well as abstracted signals (like 'pain') we have signals which measure our environment more directly (like 'hot' and 'cold'). These signals raise our fitness because they let us know not only that we should take action, but in what direction that action should be taken.