They are the nutrient cyclers in the soil food web along with flagellates, ciliates, nematodes and micro/macro arthopods. The stellate amoeba eats bacteria by surrounding it and engulfing them in gas bubbles. After eating the bacteria it releases the extra nitrogen that is not needed for their being. Making the nitrogen plant available in the form of ammonium. The nitrogen form, ammonium is great if you are trying to grow perennials, they enjoy the acidic environment that ammonium creates.
I found this guy in one of my clients soil, while I was doing her assay. How fascinating to see this creature :D