The relationship between plants and the fungi and other organisms that decompose them is very important. If the leaves and deadwood were not broken down, then we would all slowly disappear under piles of un-rotted materials, and the planet would slowly run out of the building blocks to make new animals and plants. It is thanks to fungi that the leaves and wood are broken down and recycled so that plants, and animals, can use those materials again.
The numbers are mind- blowing. It is estimated that plants on Earth add about 56 400 000000 metric tonnes of Carbon to land ecosystems every year. To keep all this in balance, the same amount has to be decomposed back to carbon dioxide water and minerals, and a lot of this is done by fungi like the one in the picture.