It really depends on what kinds of bacteria you are talking about.

As you surely know, there are also bacteria that cause diseases in human beings and well as other living organisms.

Well, one of the uses of some bacteria is that they help in digestion. In the stomach of herbivores for example, you have some bacteria which help digest cellulose by secreting the appropriate enzymes.

Maybe something else that you might know is the use of bacteria to produce yoghurt or cheese. They digest some of the milk used and produce an acid called lactic acid which curds the milk.

Maybe this link will help you.

Bacteria - Wikipedia, the free encyclopedia