But, not studied enough, symbiotic fungi offer a better "rooting" system than roots themselves. Many botanists think this symbiosis is how many plants can only survive in the wild. HOWEVER!!!!!! These fungi subside in ALL areas of the world in the soil. The main reason is to add a plethora of species is in hope of 2 things: (1) the plant does goes better with a specific myco. (2) Over-seeding (not seeds actually haha) will out-compete and prevent the bad kind of fungi. Some companies suggest these fungi should be added often for these reasons, however, once added (or already incorporated naturally) the fungi will live, multiply, grow, and most likely never have to be applied more than once except under some conditions where the bad kind of fungi is known to have infested the area.
P.S. I do add mycorrhizae to my soil and to my plants' roots directly. But my garden substrate is super poor.