What is meant by transpiration is a necessaru evil?

In plants, transpiration is a process which is inevitable but potentially harmful, because it can cause loss of water. Loss of water can lead to wilting and eventual death of the plant. Even a little stress can interfere with the plant's growth process. 

But, at the same time, transpiration is necessary because of a number of reasons - the transpiration stream aids the processes of mineral absorption, water absorption, energy exchange, helps provide the plant with optimum turgidity, it helps the exchange of gases and so on.