Question

Why is the transpiration important for plant?

Answer 1

 the evaporation of water or loss of water in vapour form, from the exposed parts of a plant especially leaves is termed as transpiration. 
transpiration is important because 
1 removes excess water 
2 develops root system 
3 gives a cooling effect 
4 absorbs and distributes salts 
5 ascent of sap  

Answer 2

Transpiration is important to the plant to produce a cooling effect to the plant at hot conditions and to enable the transport of water and mineral salts from the soil to the leaves.  

  A plant uses almost 95% of its energy to facilitate or enable transpiration, a process through which "water in the plant" is evaporated as water vapour through stomata.

  This is useful to sustain the plant in hot conditions and produce a cool environment around it.  

  Also, it is necessary for the capillary action that moves that "water and mineral salts" from the soil to the plant.