"The second light of Advent is the light of the plants; plants that reach up for the sun and in the breezes dance."
Plants remind us that everything we love about our world depends on the light of the sun. There is nothing we enjoy or need to live without the sun. And the plants' worship of this star is a reminder to us to appreciate that outer light even as we find ourselves in darkness.
Also, we are nothing without plants. Even if all you eat is meat, meat traces it's own survival back to plants.
Plants cleanse our air; they are the lungs of earth.
And, plants are pretty. There's nothing like a swath of parking lot to parch your eyes and your soul. Hans Christian Anderson said green was good for the eyes; I think he was right.