Bacteria can be beneficial to us and can also be really harmful. Some of the natural applications of bacteria include nutrient fixation in soils, natural cleaning of surface water bodies like rivers and lakes, formation of yogurt or curds, etc. We also have engineered applications of bacteria in the pharmaceutical industry for production of various chemicals, treatment of wastewater in sewage treatment plants, cleanup of soil in bioremediation schemes, etc. In most of these applications, we use the metabolic capabilities of bacteria, in which they consume a certain substrate and generate some product, to our advantage.
Bacteria can also act as pathogens and cause diseases and other adverse health conditions. Some of the common bacterial disease include, bubonic plague, pneumonia, tuberculosis, cholera, syphilis, etc. Bacteria also cause biofouling of membranes and also cause degradation of metals, including ship's hulls. This is called bio-corrosion. Health, bio-corrosion and biofouling cause billions of dollars of loss every year.
Hope this helps.