There are several reasons why people participate in clinical trials. Some people participate in order to get a new and otherwise unavailable treatment. Some people participate in order to get free laboratory tests. Also, one of the most common reasons for study participation is that it helps scientists find better treatments for HIV.
On the other hand, there may be reasons not to participate in a clinical trial. For instance, you may be asked to stop taking other medicines that may be helping you. The treatments in the trial may have side effects, or be unsafe. Other treatments you may want to take in the future may not work as well because of drugs you took in a clinical trial. Or you may just not have the time and energy that a study may require.