The primary function of bacteria is to assist in decomposition. Without bacteria to break down flesh and plant matter, our world would become overcrowded with the corpses of the dead. We would be forced to burn everything that died, which would really be a major waste of resources. As on_the_hill said, without this breakdown, we also wouldn’t see nutrients return to the soil, meaning that we couldn’t grow food or raise animals to eat. Bacteria are one of the most basic foundations of our ecosystem, and are important for our survival.
While it’s true that certain bacteria can cause problems, that’s generally when they’ve wound up a in a place they weren’t intended to be. You should remind / tell your nephew that we have dozens of types of bacteria living inside of us that are responsible for keeping us healthy; for example, without the bacteria in our stomachs and intestines, we wouldn’t be able to break down tour food for nutrients and waste disposal.