You've learned that decomposers are a special kind of heterotroph, and bacteria are important decomposers. If bacteria didn't break down organic matter (garbage) and recycle the chemical elements in the substances they break down, we could not live here. There would be too much waste and not enough nutrient and food material available for organisms on the bottom of the food chain (like plants). So, the bacteria recycle the dead stuff back into the soil to start the cycle again.
In addition, you've learned that decomposition in oxygen-poor environments can cause the production of methane by anaerobic bacteria. The methane is being used to produce electricity in some cases.