Why do we need to conserve the planet's biodiversity?

If a really serious plague or virus comes along, we need to have a diverse planet so that there may be certain plants or animals that have natural immunities to the plague. This is a major concern about America's crops because they're all the same strain. If they were all different strains not all would be lost if a plague came along that destroyed the prominent strain. Maintaining biodiversity is important for that reason.