Because you may have killed most of the bacteria but left a few with some resistance to that type of antibiotic. In classical evolutionary terms, you’ve thinned

Because you may have killed most of the bacteria but left a few with some resistance to that type of antibiotic. In classical evolutionary terms, you’ve thinned the herd to the strongest members. These surviving somewhat resistant bacteria, even if they aren’t making you noticeably sick, may then spread those genes into the general population. It’s important not to use antibiotics if you don’t need them, and to complete the course if you do need them. Otherwise you’re selecting for strains of disease that are hard to cure.