The stomata are primarily meant for absorption of CO2 but these also help in exchange of gases, but at the same time water vapors also escape through stomata. Thus transpiration is described as necessary evil because it is an inevitable process but potentially harmful. Loss of water can lead to wilting, serious desiccation, and often death of a plant, if there is shortage of water. There is good evidence that even mild water stress results in reduced growth rate, and reduction in yield.
However, transpiration is beneficial to the plants in several ways.
1) Mineral Absorption: Minerals absorbed in water are absorbed into the roots; move up through the plant in the transpiration stream.
2) Optimum Turgidity: In some plants if transpiration is not allowed to occur, plants become very turgid, do not grow well and there is shortage of water in their cells.
3) Energy Exchange: When water is evaporated from the exposed surface of cells of leaves, it exerts a cooling effect on plant.
4) Effect on Growth and Development: Transpiration is a necessary factor in the normal growth of some plants e.g. pear, sunflower.
5) Absorption of Water: Water is conducted or transported in upward direction in most tall plants due to transpiration.
6) Exchange of Gases: Wet surface of leaf cells allow gaseous exchange.