The roots of your vegetable plants can only reach so far. Mycelium in the soil helps to bring nutrients to your plants roots from several feet away.
Check out what Nathan from StPete.LocallyGrown.net has to say about it in this video.
It is easy to get some mycelium going in your garden. Just get some cardboard, or wooed chips and keep them wet. Pretty soon you will see these white strands growing underneath. That’s the mycelium. Then just move it into the soil around your garden. It will take off from there. It just needs to be kept damp.
Wherever the mycelium touches your plants roots, it exchanges phosphorous for sugars from the plant.