These microbes get their energy, carbon, and nutrients from this dead stuff. However, there is a great deal of competition between microbes and plants for these scarce resources. This residue is typically left over because it is chemically very complex and hard to decompose. However, this soil organic matter is extremely important in ecosystems because of its interaaction with soil minerals. In fact, microorganisms do most of the digestion for the herbivores.
All animals are inefficient digesters.
Thus, what they don't completely utilize for their body building and maintainance functions, the eliminate as waste. In some ecosystems this is an important flow of nitrogen to the soil organic matter. If soil erosion is severe enough, some mineral nitrogen may be lost with the soil debris. These losses are mostly a result of microbial activity and can be very important. But, this is a complex set of processes and beyond the scope of this discussion.