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.

It's cell theory. easy name i would think.