It is tempting to give a lot of value to hearsay. If you know someone who found a medication helpful, you are likely to believe it is good. However, we are often wrong when we jump to conclusions like this and trials are run to make sure that decisions are based on something much more solid than hearsay.
Trials allow us to find out about the safety and efficacy of treatments by using rigorous methods. If they are well designed and powerful enough, they give us the tools to predict how a treatment will affect whole populations including you, rather than just that someone you know.
For a long time, health professionals and patients had very little to go by when making decisions about care. Decisions about treatment were often made based on the limited knowledge and experience of an individual doctor, rather than data about large numbers of patients.
Fortunately since the 1960s, an ever increasing body of knowledge has been developed. Many trials have made an impact on the decisions made about your care today.