Why GI pipe is used from Chiller to Cooling Tower and MS pipe between Chiller to AHU?

GI pipes are used to avoid erosion and wear inside the pipes, and the fouling factor. because the cooling towers network is mostly open network ( due to Dissolved  Oxygen, salts, Acids,....)

For the chilled water network it is closed loop and you can find chemical station mostly which works on controlling water factors that effecting the life time of pipes and equipment and in the same time keep the the heat transfer factor for pipes constant as much as possible  

For the chilled water network it is closed loop and you can find chemical station mostly which works on controlling water factors that effecting the life time of pipes and equipment and in the same time keep the the heat transfer factor for pipes