What role can fungi play in plant life if they convert organic residues into substances available to plants?

For some plants, it is necessary to coexist with the mycelium of the fungus. This is a mutually beneficial symbiosis, which is called mycorrhiza. Without the fungus, the plant simply cannot exist, since it is the mycelium of the fungus that is able to convert the remains of organic matter from the soil into an available state for absorption by the root system of the plant. The mushroom gets water. There are enough examples of mycorrhiza in nature, for example, orchid, heather, blueberry, lingonberry, beech, oak, birch, aspen and mushrooms that have received names from a neighbor plant, for example, boletus, boletus.