Let's explain the reasons one by one:
We need higher thermal efficiency for better performance of the vapor power cycle.
We need the least amount of moisture at the turbine exit. Otherwise, excess moisture will erode and damage the turbine blades and decrease the performance of the turbine.
To increase the thermal efficiency we try different methods like increasing boiler pressure, reducing condenser pressure, superheating, etc.
Thermal efficiency of the thermal power cycle in general is given by, η = 1 - T2/T1, where T1 = temperature of heat addition and T2 = temperature of heat rejection
The mean temperature of heat addition should be as high as possible to increase the thermal efficiency of the vapor cycle.
Superheating is limited by the material properties so it is not the best option (although it is one of the better options available)
7. If only boiler pressure is increased it increases the moisture content at the turbine exit.
8. Reheating solves both of the problems where steam is reheated in stages.