Magnified, soil mites are lobsterlike critters (like crustaceans, mites are arthropods) with impressive armor and scary chewing parts. Their job, along with the above-named allies, is to break down organic matter (like leaves, stems, insect corpses, and dung) into a soluble form that can be taken up by plants.
How crucial are they? Let me quote from a book with an amazing title: Life: A Natural History of the First 4 Billion Years of Life on Earth by Richard Fortey. Fortey is a paleontologist, and in a scoop of soil he can find mites hardly changed since the creatures of the Devonian Age. He is also a gorgeous writer:
If the spring tails were to undergo a mysterious demise, together with the mites that live in soil and the minute fungi upon which they feed, quite soon there would be an ecological crisis of a magnitude we can scarcely imagine. Nutrients would become locked away, the soil would become progressively impoverished, larger plants would die, and soon animals would follow suit. Novels that seek to portray post-holocaust worlds always seem to assume that the soil will magically survive, and that a bean cast into seared soil will quietly proceed to a successful crop. But the soil is not a passive medium; it is alive. I doubt whether there would be many readers for a post-holocaust novel that was concerned with the hero's desperate search for mites. Bu alas for the world if the mites and their diminutive allies failed to prosper!
Since the earth emerged from the sea, there has been a mutual dependence between plants and animals; the tiny animals feed the plants. The big animals, including us, eat the plants. Plants' roots, it turns out, aren't passive participants. They exude sugars, acids, and other compounds to attract or repel bacteria and fungi and stir up microbial action.
Autumn is the best time to keep the helpful creatures warm and fed, because they're probably at their most populous. The ground will retain warmth late into autumn, and the creatures will continue to decompose the organic matter. Autumn soil prepared with a couple of inches of compost and a couple more of mulch will protect the organisms from temperature extremes and keep those organisms active long into winter.
The best down quilt to lay on your planting beds is compost. Next-best: leaf mold from last autumn's leaves and, after that, this year's shredded leaves. And if you want to just go out and buy something, get bags of shredded bark.
One caveat: Don't use oak leaves. Part of the mighty oak's mightiness is that the leaves are slow to break down—they'll make a layer like Naugahyde. Other big leaves, like maple and tulip poplar, mat together and in northern gardens can freeze into a giant pancake that keeps water and air from the soil below.
You want to build your soil rather than dose it with fertilizer. Scary though the thought is, you actually want to participate in the cycle of decay and rebirth. The nutrients you give the critters go into the roots and up to the leaves. They return to the earth this time of year, when the leaves drift back down. Since we're all quoting FDR these days: Don't leave those all-important soil creatures ill-housed, ill-clothed, or ill-fed.