Transpiration is supposed to help the plant in the following ways.
1. Excess water getting into the plant might decay the cells. Transpiration prevents it by removing the extra water.
2. Extra quantity of water, if retained would disturb the osmotic relationship between cells. This is prevented by transpiration.
3. Transpiration stream helps in the distribution of nutrients to all the parts of the plant body.
4- Fresh and cool water reaches all the parts of the plant body and this reduces the metabolic heat acting as a coolant.
5- Transpiration indirectly helps in the uptake of salts as the latter get into the plant together with water.
6. Upward movement of water (Ascent of sap) is due to the suction force created by transpiration.
Some of the recent researches however, have indicated that transpiration after all is not advantageous as was thought previously. The fact of water escape is a consequence of the anatomical construction of the leaf. The anatomical structure of the leaf is necessary for photosynthesis and respiration. When there is an escape route water also escapes. Transpiration does not remove excess of water. The plants in fact are forced to absorb more water because most of it will be lost by way of transpiration. That is why transpiration is often called a necessary evil.