If we believe the world to be divinely created by a loving parent (or parents), and if we believe we are souls that, with the help of God, chose to incarnate into the world for the purpose of spiritual development and learning, then Leibniz's "best of all possible worlds" declaration begins to sound less and less outlandish. We chose to come into the world at a specific time as incarnated individuals, under unique circumstances, and with the potential of unique experiences tailored specifically for us and our spiritual development, experiences that include suffering and evil.
Though Voltaire saw no redeeming qualities in the evil and suffering his characters encountered in the novella, evil and suffering can, in fact, have redeeming qualities for us as individuals if we view these negatives through the lens of our spiritual development. Of course, we cannot always consciously know what these redeeming qualities are, nor should become Panglosses and make attempts to find the good behind every cause in this world because there may not actually be any residual good that comes from a specific episode of suffering or evil in this world. The 'good' an episode of evil and suffering contains may reside entirely outside of our world.
Voltaire does not consider these possibilities at all in Candide, which is why his criticism of Leibnizian optimism, though overwhelmingly convincing and entertaining at the surface level, becomes increasingly strained when deeper metaphysical considerations are included in the mix.