A garden is only as good as its soil! Many first-time gardeners assume that you can just plop a plant into any old soil, and it’ll grow beautifully—and that might be the case if you’re lucky enough to have nice, fertile soil. But most of us have to amend our soils for both nutrients (adding in the goodies our plants want to “eat”) and condition (the way the soil feels—heavy, clay, sandy, loamy, etc.).
Mulch is used to retain moisture in the soil, suppress or block weeds, keep the soil and plant roots cool, prevent frost heaving in winter, and make the garden bed and landscape look more attractive.
While you may have to readjust stones every so often because they sink into the soil, you won't have to worry about your garden bed eroding in harsh weather conditions. Basically, rocks give soil something to hang onto, which is very useful in heavy rains or times when there's significant water runoff.