They'll help the new stuff get started. Hot pile Aerobic microbes chow through a pile of organic matter quickly, heating up the pile as they decompose. Aerobic microbes need oxygen to work. We gardeners can think of them interchangeably. Loam Soil with a balance of fine clay, medium-size silt, and coarse sand particles.
Most plants grow well in it, so most gardeners want it.
Feed compost to the microbes in your garden's soil and you will have soil with most of loam's desirable properties. She spreads out leaves, grass clippings and other compost materials in layers right on top of the soil. Kind of like lasagna. She works the layers into the soil, and they decompose underground. This is a good answer for gardeners who don't have space to set up a compost pile.