Why bacteria is called useful as well as harmful

It’s because there are bad and good types of bacteria for example when you get an infection the doctors will perscribe you antibacterial meds .. in the meds is good bacteria the good bacteria you take through this medication will fight off the bad bacteria to go back to maintaining the normal balance of good and bad bacteria same goes with acne meds there is acne types which are caused by bacteria in the skin if you go to a dermatology the first thing they will try is birth control if that doesn’t work there’s a couple options left but they’re next try is an antibacterial med as well n it will fight off the bad bacteria in your face n bring it back to its normal healthy balance