We use plants mostly for food. Any industrial uses for plants would compete for land that could be growing food, or for food itself. Despite that, there are many industrial chemicals that are made from plants.

An example is the alcohol that is added to gasoline. Normal economics would prevent corn from being used to power cars, since it is more valuable as food. But governments pay distillers to make alcohol from corn, so that corn prices will be higher, and benefit the corporations that grow the corn.

There are many non-food products that can more cheaply be grown than manufactured. Carnauba wax, candelilla wax, jojoba oil, gum arabic, gum tragacanth, and natural rubber are just a few.

But where plants are most useful as sources of chemicals is in medicine and pest control. Because plants make many very complex molecules that are very hard to make synthetically, many medicinal proteins and drugs come from plants. Roughly a quarter of all prescription drugs are derived from plants.

Plant derived insecticides and insect repellants are another class of molecule that is cheaper to get from plants than to try to make in a lab. These also have the benefit of being easily biodegradable, so they don’t linger in the environment.

Plant derived dyes are another class of chemicals that are cheaper to grow than to make. Carotenoids (the reds, yellows, and orange molecules in autumn leaves) are widely used in industry. Indigo blue, the browns of henna, the yellows of saffron and turmeric are other examples.