Been studying these beasts for two years.
They are herbivores, mostly feed on dead plant materials. I have been keeping them around my garden, and none of my plants have suffered.
More noticably, I can say, I have seen it eat whiteflies, aphids, and other insects which my dwell in the soil, or make habit of staying in the soil in any fashion.
My studies have been focused on mainly sow bugs and millipedes.
before dead plant material gets to the stage where the mites and springtails can work on it, it has to be shred into small chewable bits. This is done in our forests (and nearly all other ones as well) by millipedes and sowbugs. We need to know whether these critters prefer different kinds of plant litter to eat (deciduous leaves versus conifer needles), how fast they grow and what precise chemical changes they accomplish to the nitrogen in the litter. (for instance, sowbugs have a calcium skeleton (like us) -- maple leaves are high in calcium; is that important?)
We have 2 very common (+2 not so common) introduced species of sowbugs in the area -- which are starting to invade the forest habitats as well. I don't know whether they eat conifer needles at all, etc, etc. (if they do eat conifer needles, then their invasion may entirely change forest ecology in the NW). We also have 1 very common native species of sowbug that feeds primarily on dead wood, but also likes to feed on litter as well (when? why?) The native species lives in very wet environments, and may therefore excrete nitrogen in liquid form. The introduced species (even though they actually have gills and not lungs) live in much drier environments and probably excrete waste nitrogen as insoluble uric acid (bird poop). Liquid nitrogen can be immediately recycled by the plants and fungi; insoluble nitrogen is a different matter. We have no idea how it cycles -- minutes, days, months????.
I've been using MY garden, and several other captivities, and a forest, which my professor owns, to study the habits of Millipedes and sow bugs
So collectively, if you trust these bugs(which I do) they can in return, provide a lot for your plants.
Mychorrizal fungi, which produce enzymes that make nutrients available to the plant, grow into the root system. They help supply nitrogen, phosphorus, boron, and water to the plant.
The presence of the 2 bugs I am studying are responsible for aiding mychroizzal fungi, and producing other key compounds for a complete food/nutrient web in any soil ecosystem