This Creepy-Looking Fungus Known as "Dead Man's Fingers"

Dead Man's Fingers are indeed wood-rotting fungi, but they specialise in consuming neither the softish cellulose nor the much tougher lignin but rather the polysaccharides - glucan and other minority content compounds of timber that bind the cellulose and lignin together to form what we recognise as wood. As a result, when these and various other ascomycetous fungi have consumed what they can of a dead stump the remainder is a nutrient-rich soft mess that insects and other small creatures are able to feed upon (if other cellulose- or lignin-rotting fungi haven't found it first).