Saprophytic fungi are the largest group of (macro) fungi, responsible for breaking down and recycling dead plant and animal material. These are the fruit-bodies you see on dead trees, leaf litter, animal bones, even faeces.
Saprophytic fungi release enzymes to break down and digest the lignin, cellulose or chitin in this material into simple soluble compounds that can be absorbed by them, and by plants, as nutrients. In so doing, they play a vital role in reducing the accumulation of dead organic material and in recycling essential nutrients, particularly carbon and nitrogen.
Without these digestive activities, forests would disappear under a mountain of logs and leaves!