What is the use of bacteria?

The benefit is great:
They are not replaceable in medicine. By implanting human DNA into their nucleoid, there is the ability to synthesize insulin, which is used in medicine
In industry, one cannot do without them: all fermented milk products are their merit
They also live in our intestines: bifidobacteria, Escherichia coli, etc.
Play a role in nature’s food chain
Well, they probably reduce the number of animals – including humans, those that harm us – natural selection).