The first category of constructive purpose is to help the person about whom one is speaking. There is a tendency among many to ignore the misconduct of others in favor of a “live and let live” attitude. Let us first understand why the Torah rejects this attitude.
(1) Every person is born with character flaws. It is our mission in life to change, to grow, to strive for perfection and spirituality. We all want to rid ourselves of our imperfections; people do want to be good. What makes life so challenging is that negative character traits are blinding. We either lose sight of what our goals should be or rationalize to the point that we simply cannot differentiate between right and wrong.
(2) One of the greatest gifts Hashem has blessed us with is companionship: friends, family, people around us who are close enough to care, yet distant enough to be objective. To abstain from speaking up and offering one another reproof and guidance amounts to depriving one another of one of the most valuable tools for personal growth. We must get involved and, when necessary, even enlist the involvement of others in helping people through the struggles of life. Remaining silent when reproof is called for is not shmiras haloshon, it is depriving one’s fellow of his lifeline to self-improvement.
Sefer Chofetz Chaim, 21 Cheshvan, page 150