Some, such as mushrooms, are good to eat, but others are deadly poisonous.
Yeast helps bread rise and beer ferment.
Living off others Fly agaric is deadly. Eating even small amounts can knock you out. Although they grow in the ground like plants, fungi are not really plants because they have no chlorophyll and so cannot make their own food. Because they cannot make their own food, fungi must live off other plants and animals. Fungi are the biodegraders of the living world.