What is the Purpose of Water Bath in a Rotary Evaporator?

What is the Purpose of Water Bath in a Rotary Evaporator in order to separate and remove the undesirable impurities. The heating of water and the convection of heat by means of a fan plays an important role in achieving thermal contact between the hot air and the evaporating liquid at the same time making it possible for water to be converted into vapour and be carried away by vacuum tubes. By applying vacuum, water vapour is collected or condensed on condensing surfaces which will be removed from them after cooling down.