Although there is never any guarantee that an investigational medicine will be beneficial (and often not all of the trial participants will actually receive it) a clinical trial may be seen as an important option if there are no existing treatments for a particular condition or if currently available treatments have not worked. This is especially true in certain medical fields like cancer.
Not all clinical trials look at potential new medicines – some look at potential new uses of existing medicines or test out other theories about how to improve healthcare. Some people join trials like this because they have a particular personal interest in the question the research is trying to answer.