We need conviction because it empowers us to act. Without it, we stare out the window doing nada. Indeed, great strategy is based on conviction.
But conviction and certainly different kettles of fish. Certainty precludes investigation. It closes the mind and forces one into a defensive intellectual posture. If you are “certain”, for example, that it is 2;00, you will tend to argue with someone who says that it is 1;55.
By way of contrast, conviction implies sufficiently strong belief to act. It is less than certain until the action (as a test) produces a result that either confirms or contradicts the conviction at hand.