Fungi use Mutualism, one of the Mechanisms of Facilitation to aid plant growth. Not only Nitrogen Fixing but as stated on the linked blog post, Phosphorus uptake. In my studies for Polyculture Design, MoF are key to developing a sustainable plant guild. Building the guild involves root inoculation of the plants with mutualistic flora. Just like planting the right companions above ground , the correct underground companions work for plant health too.
Permaculture Reflections: Increased Drought tolerance and Resistance to Salinity Through Fungi