Surviving a massive destruction is not always merely an act of divine mercy. Sometimes, God leaves a small remnant of people alive to serve as living witnesses to justify His judgment in the world. A very small, easily countable number of individuals will survive while the rest of the nation perishes through war, hunger, and disease [מצודת דוד, ביאור שטיינזלץ].
The survival of this small group serves a distinct purpose. When they are taken into captivity in foreign lands, these survivors will openly speak about the terrible deeds they committed and the punishment that followed [מצודת דוד, ביאור שטיינזלץ]. The primary approach among commentators is that this retelling proves the destruction was not without reason. The survivors will admit their guilt and acknowledge that they suffered directly because of their own sins [רש"י].
However, there is another perspective regarding the nature of these survivors. Some note that the surviving group will include wicked individuals who, even in exile, refuse to truly recognize God. Yet, simply by speaking about their awful actions, they will unintentionally fulfill the divine goal of publicizing the strict justice of their punishment [מלבי"ם].
The ultimate result of this entire process is a profound recognition of God. While some explain that this new awareness will become general knowledge [ביאור שטיינזלץ], most commentators emphasize that the true target audience is the surrounding nations. As the nations of the world observe the exiled survivors living among them, they will recognize that God punished His own people fairly. This will prove to the world that He actively oversees the entire earth and faithfully delivers appropriate consequences [מצודת דוד, מלבי"ם].