The main reason to attempt to stop aging is to prevent other illnesses from taking effect.

People who are physically young are at a much lower risk of cancer, cardiovascular disease, Type II diabetes/Alzheimer's, etc. Stopping/reversing the aging process kind of fixes the root cause of a lot of those diseases.

We're probably not far off from that reality, either. Then we move onto more controversial questions, e.g. Do physically healthy people have the right to end their own lives? Not everyone want to live to be 300 years old, especially those who have lost most of their family and friends over those years.