By virtue of their chloroplasts, plants are superb harvesters of solar energy. They use it to build leaves, flowers, fruits, stems, and roots. We harvest a small percentage of that energy in the form of food and a smaller amount in the form of wood for heating.
But the vast majority of plant biomass goes unused by us. In every agricultural region on Earth, huge amounts of the structural parts of crops -- things like cornstalks, sugar canes, beanstalks, and wheat stems -- are discarded because we haven't figured out a way to convert them into fuel.