During the last stages of the steam turbine, the steam pressure and temperature is low. What would happen if the temperature of steam at the last stages fall below the saturation temperature?

Answer 1

As the saturation temperature is defined as the point where a phase change occurs (vapor to liquid or vice versa), your vapor (steam) would condense into a liquid (water).

While that might seem fine, the newly formed liquid would interfere with turbine operation. First, unless it was "filled" with water, those droplets would not exert force on the turbine blades to generate power. More importantly, the force of impact of water droplets would dent and deform typical turbine blades (think of your car roof when it hails).

Those dents would interfere with flow (just like dents in an airplane's wing), and thus interfere with turbine performance.