Question

Liquid water is sprayed into the hot gases before they enter the turbine section of a large gasturbine power plant. It is claimed that the larger mass flow rate produces more work. Is that the reason? 

Answer 1

No. More mass through the turbine does give more work, but the added mass is only a few percent. As the liquid vaporises the specific volume increases dramatically which gives a much larger volume flow throught the turbine and that gives more work output.

This should be seen relative to the small work required to bring the liquid water up to the higher turbine inlet pressure from the source of water (presumably atmospheric pressure).