There are many reasons why rainforests are important which can be put under the following headings: Biodiversity. Rainforests contain about half of the existing plant and animal species in the world.They contain a third of the world’s bird species and 90% of its invertebrates.The remaining African rainforests contain more animals and plant species than are found anywhere else on the continent.ainforests contain about half of the existing plant and animal species in the world. They contain a third of the world’s bird species and 90% of its invertebrates.