Mycorrhizae is a type of fungi prevalent all over the world in plants root systems. This type of fungi develops a symbiotic relationship with the roots of many plants, spreading out in hair like structures that increase the plants ability to uptake nutrients. They also have been shown to break down dead organisms and minerals, making them bio-available to again to your plants.
Similar to fungi, beneficial bacteria has a symbiotic relationship with your plants. Plants release exudates from the roots, which feed microbes that in turn break down minerals and other materials so they are absorbable by your plants root system. Soil grower's will find healthy microbiology imperative to a successful grow. Many soil grower's will enhance the microbiology in the soil using compost teas, while synthetic grower's often prefer bottled concentrated microbes. Whatever your grow style it is likely to benefit from these beneficial relationships available from fungi and bacteria.