Fractional Distillation
It simply spreads the temperature change over a longer distance, enabling separation of distillates with boiling points close to each other. "Generally the component parts boil at less than 25 °C from each other under a pressure of one atmosphere."

Imaging reading a mercury thermometer one cm tall as compared with one 100 cm tall: it would be difficult to distinguish temperatures on the smaller one. In the fractionation column below, it helps keep the outputs separate.