Transpiration is the process by which plants lose water in form of water vapour from aerial parts of the plant. When there is a huge amount of transpiration that takes place, it leads to huge amount of water loss thus causing wilting of the plant. But transpiration is needed for the pull of water and minerals from the root to the other parts of the plant and is needed for stomatal opening for the purpose of photosynthesis and respiration. So though it is a necessary evil and might also cause death of the plant if excessive water is lost, yet this process is necessary to carry out the other processes and for the need of water and minerals which is why transpiration is considered a necessary evil in plants.