If it were not for some insects, many kinds of plants could not have seeds. If many plant-eating animals did not have natural enemies that eat them, the plants would be destroyed. Plants and animals need each other Plants and animals trade with each other. They trade for things they both need to stay alive. A butterfly sucking nectar from flowers of the Coat Buttons.
Animals breathe air.
The air gets changed inside their bodies. When they breathe the air out again there's something called carbon in it. Animals cannot use their air again. But plants need carbon. The plants trade fresh air for the carbon they need.