Why plant plants?

Some people ask why plant plants? Well theres a simple answer to that, imagine the world without oxygen. How would we live? How could other animals live? And why does earth have oxygen but other planets dont? Vegetation creates oxygen. Say if we cut down all the plants in the world and still had pollution in the air from coal plants, what would happen? We need to have oxygen to support all of human life, and that helps us by planting plants. Plus, who's to say that the plants don't look nice? They look great and can help clean air stimulation.