Plants are the primary producers in an ecosystem, and thus the cornerstone of the food web. They capture light from the sun and are the starting point through which energy enters ecosystems.
In a healthy, natural ecosystem, plants are eaten by a wide range of insects and other invertebrates, as well as by larger animals. Mammals eat the leaves of plants, and birds and other animals eat fruits, seeds, nuts, and other parts of plants. But birds and insect predators also prey on the small insects and insect larvae that eat the leaves, stems, roots, and juices of plants.
Plants and insects co-evolve, and insects are often very specialized in what they eat. When people introduce plants to a new area, they typically don't bring any or all of the insects and other animals that eat the plant to this area, and often, the plant won't be eaten by as many native insects.
With fewer or no local insects or other herbivores eating the plant, the plant creates a "dead zone" in the food web, reducing insect biodiversity and indirectly reducing the diversity of birds and other larger animals as they have less insect food to eat.
By planting native plants, you help protect and restore this biodiversity by supporting the whole food web!