What is the importance of plants to human beings and animals?

Animals eat plants (of eat the animals that do). Animals can't make their own food from energy and simple compounds, and even if they could, animal energy use is too prodigious to support by photosynthesis of the organism itself. A 70 Kg human needs around 2,000 - 3,000 kilocalories a day; to make that requires several hundred Kg or plants and many days. It requires a field of really good grass the size of a Rugby pitch to keep one cow alive. Short answer: plants are vital; if they die, we die.