Adds to the insight of the person being criticized
Constructive criticism can help the subject increase his/her knowledge base and insight. Consider the example of a editor and a writer. If the editor keeps on pointing the weak points of the writer in an amicable, it will only be a matter of time when the writer will learn all the tricks of the trade, and will begin to give flawless articles. On the other hand, if the editor avoids confronting the writer, he will get into a comfort zone and then it will be very difficult for him to break the improvement barrier.
Your constructive criticism may not be received well in the beginning, but when the person in focus starts to reap its benefit, he will realize your effort and concern. It will be then, when he will start to trust you and be able to develop a close working relationship.