The turbine referred to, in your text book must be a specific type of turbine, namely reaction turbine. Reaction turbines utilise mainly the pressure energy of flowing water to produce Power. The flowing fluids consist of both pressure energy(a function of fluid pressure) and kinetic energy(a function of flow velocity). The reaction turbines are rotated by the pressure difference of water on both sides of turbine blades. The reaction turbines convert most of the pressure energy to mechanical power, but they are unable to convert much of the kinetic energy in such a way. So the kinetic energy brought in by the water is left to flow away through the outlet
But if a pipe whose diameter is small at first and then increasing slowly is used at outlet, then the pressure at the pipe beginning will be lowered significantly. This happens due continuity conditions and Bernoulli's principle.
According to continuity equation if the cross sectional area of a flow decreases the flow velocity increases. But according to Bernoulli's principle if flow velocity increases the pressure decreases, given there is no change in elevation, losses, external work etc.
Thus the pressure at outlet side of turbine decreases and the pressure difference between both sides of the turbine increases. Thus the driving force increases and hence the power. So such A pipe actually converts some of the kinetic energy into pressure energy for better utilisation in reaction turbines.
Now, the water cannot flow outside if its pressure remains below atmospheric pressure. So in order to increase the pressure the diameter is increased slowly. In this case also Bernoulli's principle is utilised. The increase in diameter of the outlet pipe is kept gradual to avoid losses (eg: Eddy formation due to sudden expansion). Such pipes with gradually increasing diameters used in reaction turbines are called Draft Tubes.