How do plants support the existence of life? Why, just about by doing everything! Think about this... If you got rid of all living organisms that are not plants, the plants would still survive and develop their own ecosystems. However, if you got rid of all plants, nothing else could survive!
Think of the basic "sun feeds grass, cow eats grass, people eat cow". That grass is a very vital part in every animals' life - including us. Everything people eat comes from a plant based substance. Vegetables, fruits, beef, chicken, lamb, milk, eggs...you get the idea. The fruits and vegetables you eat are actually the seeds or the 'embryo' of the plants, they create fruits and vegetables to make the seeds more appealing, or to protect the seeds until they get to a more suitable location. The meat that wea eat - well, we dont eat many carnivores, do we? The majority of the meat that people eat comes from herbivores or sometimes omnivores. Milk, eggs, honey? All from plants. If the animals who produced those goods didn't have their respective plant life they needed, they could not create those goodies we enjoy so much.
Atmosphere. Back before people began all the polluting they do now, the atmosphere was pure and clean (save for the methane), and this is because of plants. Plants act almost like a filter, taking in carbon dioxide (the majority of the air we breath out) and they use this to create more energy for themselves. The benefit to this is that they are cleaning up our mess. BUT WAIT - There is more. Not only do plants breathe in our waste product, they also create oxygen! This is the actual chemical chain for taking in carbon dioxide and releasing oxygen:
6CO2 + 6H2O + Energy ® C6H12O6 + 6O2
Now, plants also do "breathe" oxygen, but they only need enough to help break down complex carbohydrates formed by carbon dioxide, sunlight and water.
Plants also help to create soil and hold it in place, serving as anchors for dirt to not just blow away or wash away. Did you know it takes 100 years of undisturbed dirt to form one quarter inch of topsoil? Yeah, we need plants! While serving as anchors for our dirt, plants also provide very good habitats for the animals we depend on and for the animals we don't depend on. Deer, squirrels, horses, cows, pretty much any animal that lives. Plants are shelters when needed.
So, in addition to plants doing all of this to support the existence of life, they also provide very useful products for humans and animals to use. Just one example, for animals? The trees provide useful teeth and claw and horn sharpening for animals like beavers, wild cats, and deer. And example for humans? Well you have clothing that is made from cotton, you eat fruits and vegetables and meat, and chances are, the majority of materials that made the house you are living in is made from some sort of plant.
So now you know just how important plants are to the existence of life on our planet. Let us not ever forget this, and remember to always plant more!