Nutrient cycling

Fungi are the major decomposers in terrestrial ecosystems. The break organic matter into inorganic molecules. These are taken up by plants and other organisms. 

"Fungi have evolved enzymes that can digest some extremely tough substrates. Chitin (insect exoskeletons), keratin (skin, hair, horn, feathers), cellulose (most plant debris) and lignin (wood), nourish many fungi, though cellulose and lignin remain completely unavailable to almost all animals (except with the collaboration of microbial symbionts). Their unusual ability to exploit cellulose and lignin gives some saprobic fungi almost exclusive access to the massive quantities of plant debris produced every year, and may well make them the world's number one recyclers. Only man-made plastics are, perhaps unfortunately, immune to their attacks. » Bryce Kendrick

Along with bacteria, fungi are the main decomposers & recyclers of organic matter.  Without them, there would be no regeneration of life.  Dead plant matter and animals would pile up high and prevent further growth.

Saprophites: decomposition

Parasitic:

Mycorrhizal: Mycorrhizaae “fungus-root”