Mast provides food for animals, it produces the next generation of plants, and it also puts nutrients back into the soil as it decays. Dead trees provide homes for cavity nesting birds. Snags are home to many insects, birds, and mammals. By leaving snags you are providing these animals a natural home in which to live and raise their young. When you remove a snag the animals may look to your garage, shed, or your home for their next shelter.
Fallen logs are an ecosystem in themselves.
The decaying wood provides a growing medium for ferns, orchids, and mushrooms. Squirrels and other small mammals eat the mushrooms. Insects lay their eggs in the decaying wood, which provide food for woodpeckers and other birds. Mammals forage in the decaying wood for insects and if the use the hollow of a log for shelter. Next time you feel the need to tidy up nature, take a minute to think about all of the deprivation you will be causing.