Antimicrobial drugs either kill microbes (microbicidal) or prevent the growth of microbes (microbistatic).
The vast majority of bacteria are harmless and humans have evolved to tolerate their presence. Only particular strains of bacteria are harmful, and only if they are provided the conditions where they can reproduce to a population large enough to cause harm.
Because harmless bacteria both consume resources and take up space, they make it difficult for harmful bacteria to establish themselves. For this reason widespread use of antibiotics in the absence of specific bacteria related illness can be far more harmful than helpful.
Note that this does not apply to areas of the body not normally exposed to bacteria (areas other than the skin, gastrointestinal tract, respiratory tract, etc). For this reason activities such as surgery are carried out under antiseptic conditions to avoid infection.
There are also activities that provide a high probability of coming into contact with harmful bacteria or contaminating foodstuffs where bacteria are provided with rapid growth opportunities where handwashing is highly beneficial. Such as before preparing food, after using the lavatory, after physical contact with a sick individual, etc.