To be fair, even in the modern world, there are still many situations where a person may loose access to needed calories. For example, you go out for a bike ride, get hit by a car and end up in hospital. It may be days or in rarer cases weeks before you even eat a substantial meal again. Yes you may be given nutrients and fluids artificially, but not to the degree that we get from eating. Weight loss in hospital stay is incredibly common. There are other scenarios too where someone may need fat to survive.