Bacteria are indispensable for food digestion.
In a simplified view, food is degraded in the stomach by acids, then the bile destroys fats, and bacteria in the intestines degrade large biomolecules further into smaller ones that can pass the intestinal surface. Bacteria produce certain enzymes that can degrade biomolecules that higher organisms don't produce. Or white ants that can eat wood because of the bacteria they carry. We are just as dependent on our native microflora in our gut. Bacteria that live in a host and are beneficial to that host, at the same time being fed by that host are called commensals.