Ultimately, we ourselves would become extinct. Not only do plants produce most of our food, they also produce the oxygen that we breath. Plants take in carbon dioxide, which we breath out, and give off oxygen, which we breath in. Thus, without plants, we would really be in a rough spot. Plants also help to keep our world looking the way it is.
Plants keep the world from becoming a wasteland.
Farmers use their crops to keep the topsoil in place. This enables them to continue to use the land. Trees protect from the wind. Plants are all around us, and they shape our environment in ways we don't always think about. Next time you go outside look at the world around and picture it without any plants.