Why is steam heated before it goes into a turbine?

Steam direct from a boiler contains microscopic droplets of liquid water. This steam must be superheated to vaporize these droplets. If this is not done the droplets will pit the turbine blades and can cause premature turbine failure.

Before the development of zirconium alloy fuel pellet cladding for nuclear reactors the reactor itself could not be operated hot enough to directly superheat its steam. So early designs proposed "hybrid" reactors, using a nuclear reactor to boil the water and make steam and a fossil fuel plant to superheat the steam. But zirconium alloys were developed before any "hybrid" reactors were actually built.