One third of all plant species have edible fruit, tubers, nuts, ect. Not only the plants we eat, but the plants other animals eat in the circle of life. Food supply still depends on the plants of wild lands and traditional agricultural practice. We cannot keep going around and clearing more land full of wild species so that we can grow one species. Not only do we need plants for out food supply but also medicines.
People depend on plants for their health.
One fourth of prescription drugs come from plants. More than 3.5 billion people in developing countries rely on plants for medicine. We use to depend on plants, now they depend on us.