Calculating the entropy of vaporization at temperatures other than the normal boiling point

I would appreciate if someone could explain why this is so:
To find the entropy of transition at another temperature, we have to break the calculation down into three steps . For example, to find the entropy of vaporization of water at 25 C and 1 bar, imagine that we
1. heat the liquid to its normal boiling point, 100 C 
2. allow it to vaporize,
3. then cool the vapor back to 25 C


Why do you do these three processes? I don't understand why this would give you the entropy of transition at another temperature... 


