Vacuum manifolds, also known as Schlenk lines, are typically used in the final drying stage of samples following rotary evaporation. A vacuum pump is needed to remove any solvents or acids remaining after the distillation process.
The flow rate of the application is restricted by the size of the manifold and stopcock. If the flow rate of the vacuum pump is too high, vapor may pass too quickly through the cold trap, and not have sufficient time to condense. This would contaminate the pump, thus shortening its lifetime.