Plants grow, plants are alive. Plants are not animals.
What is a meaning and purpose of plants we can come to understand? Plants are required for animal life to exist. Plants serve the purpose of sustaining animal life on the planet. That is what they are meant to do in order to provide a habitat for higher forms of life: animals. All animal life depends on plants. Plants do not require animals to live. Plants are here to sustain and nourish animal life. If there were no plants, there would be no animals. Clearly, plants are required for animal life, hence, it is a purpose of plants to sustain animal life.