The function that provides the closest approximation to the relationship observed between the input and the output will be used to predict the value of future points drawn from a particular probability distribution (which is the goal of learning).
The function that provides the closest approximation to the relationship observed, the input, more than likely, will not exhibit a perfect relationship on a smooth mathematical curve. Even more, there will be an additional need to predict the value of future points drawn from a particular probability distribution (which is the goal of learning).