Why is superheated steam used in power plant?

Answer 1

because if the steam is not superheated the remaining water droplets reduce efficiency and can cause corrosion and pitting.

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Nearly! Saturated steam, as non-superheated steam is called, is really only steam at the pressure and temperature in the boiler. Once it starts to expand as a gas in doing its work, the pressure and temperature fall and condensation will set in, leaving less and less useful water-vapour to act as a gas. Hence, as you say, the efficiency falls.

Answer 2

Two reasons. Superheated steam has zero moisture content, thus corrosion is reduced. Secondly, superheated steam carries more energy per pound of steam without increasing the steam pressure, thus less heavy materials need to be used for the same power.