Treating illness is a broad phrase that sort of covers everything we want from a drug. Blocking pain is probably the only other reason we should take a drug.
All drug molecules affect the body, and usually in several ways. This means that even if a drug is treating your illness, it might have bad side effects too: vomiting, diarrhea, headaches and other side effects like that are usually tolerated for a good drug that can treat a disease.
This is why taking too much of any drug is bad for you- they can almost always have bad effects too. Aspirin can stop pain and treat heart disease, but in rare cases it can also cause bleeding in you stomach. As long as we are responsible and follow instructions for taking medicines the side effects should be minimal.
Drugs are capable of improving out quality of life and lifespan as clearly evidenced over the last 200 years when medicine changed from being guesswork to a scientific clinical judgement. Doctors, pharmacists and nurses are now better able to save lives and this is essentially due to drugs.