Necessary? No. Awesome? Yes.
Rotary evaporators are used to distill solvents. Distillation is the process of separating a mixture into its components based on their volatilities (how easily they vaporize). This is done by evaporating and condensing the solvent.
A solvent is placed into the evaporation flask. The vacuum is used to greatly reduce the boiling point of the solvent. This allows the solvent to vaporize just above room temperature--higher temperatures would destroy the flavor and aroma compounds. As the solvent evaporates it travels into the condenser where it re-condenses (big shock) and drips into the receiving flask.
So now you're probably thinking, how the hell would this process apply to a restaurant kitchen?
Say you were making a dessert and wanted a sauce made from blood oranges.
By running freshly squeezed blood orange juice through a rotary evaporator, you could remove the water from the juice. Left in the evaporation flask would be a thin syrup, the "essence" of the blood orange juice. Using this syrup to make your sauce would result in a product that was more "orange-y" than anything you could achieve by reducing blood orange juice on a stovetop.
Example: At the French Culinary Institute, Dave Arnold uses rotary evaporation to make the "perfect" gin and tonic. He uses the evaporator to make concentrated lime juice and distill his own gin (flavored with any aromatic conceivable). Arnold even does away with the Schweppes and injects CO2 straight into the drink.