Today I was watching Val and Eli's video on youtube. I usually watch them with the volume off while I am watching tv. This time I had the volume up. Val said something that I hadn't thought of before, but it makes total sense. As mulch decomposes below a tree or a plant, the microbes respire and release CO2 which is beneficial to the plant (paraphrased.) So besides holding water, releasing nutrients as decomposition occurs, shading out weeds, etc, it provides a necessary component for photosynthesis.