Research is how we find out how to treat diseases. When a doctor gives out a medicine for an illness, they are not just taking a wild guess at what might work. They are using hundreds of years' worth of research in to the chemical compounds that help certain ailments.
Illnesses are not static things, they change and evolve and get more complicated, presenting new symptoms all of the time. The only way that we can keep up with them is to have constant research in to drugs, both to find new medicines and to adapt the current ones. That is why illnesses that we have been suffering with for hundreds of years, such as influenza, are still being researched today. It's not that a cure can't be found, it's that the illness changes and becomes resistant to treatment. Only through research can we improve the treatment and make it effective once more.
In addition to evolving illnesses such as the flu, there are also the discoveries of new illnesses, such as AIDs in the 80s. Without previous knowledge about what works for patients with the newly discovered conditions, doctors must turn to research labs to find suggestions on what they can use.
Drugs can also have negative consequences. In the 50s, a drug known as thalidomide was given to pregnant women to combat morning sickness. When a phenomenon was seen in which hundreds of babies were born with defects, research pointed to the thalidomide being the cause. It was then withdrawn as a treatment.
Drug companies are also constantly researching how they can make their products cheaper. While this is primarily so that they can increase profit margins, it is also important in protecting more people from disease. If drug X helps 100,000 people and then the research leads to it being produced for half of the cost, health agencies can spend the same amount of money and treat 200,000 people.