Antibiotics kill bacteria. That single fact determines everything about when they are useful.
What they treat
Bacterial infections. Strep throat, urinary tract infections, bacterial pneumonia, cellulitis, and many others. In these cases they are among the most effective drugs in medicine.
What they do not treat
Anything viral. Colds, flu, most sore throats, most coughs, most sinus infections in the first week. Taking antibiotics for these does not shorten the illness by a single day.
The persistent belief that they help comes from timing: most viral illnesses resolve on their own, and if antibiotics were started partway through, the recovery gets attributed to them.
The cost of unnecessary use
Side effects, which are common – digestive upset, disruption of gut bacteria, allergic reactions, and occasionally serious infections that follow the disruption.
And resistance. Every use selects for bacteria that survive. This is happening at population scale and is producing infections that are difficult or impossible to treat.
The finish-the-course question
Long-standing advice, now debated. The reasoning was that stopping early leaves survivors. Recent analysis suggests longer courses may drive resistance rather than prevent it, and shorter courses are adequate for several conditions.
This is genuinely unsettled. Follow what your doctor prescribed for your specific case rather than a general rule.
What to ask
Whether the infection is likely bacterial, and what happens if you wait. Doctors frequently offer a delayed prescription – fill it only if you are not improving in a few days – which reduces unnecessary use substantially.
The caveat
General information, not advice about any specific illness.