Nature has well knit system of biodiversity. Microbes that thrive on dead organic matter and decompose it to its elements are a sort of sanitary inspectors of nature. Dead animals, plants, organic food remains are acted upon by such microbes.
Due to their differential activity on organic food they have also been tamed to undertake selective degradation, up gradation modification of food items turning them into cheese, vinegar, alcohol etc.
Other microbes are helpful in nitrogen fixation, phosphorus solubilization, and mineralization of elements.
Nitrogen fixing bacteria in symbiotic and non symbiotic mode are important for agriculture.