Watchful waiting is also known as expectant management. Modern usage of this term refers to observation of the patient with introduction of systemic treatment when local or metastatic disease becomes symptomatic.
Watchful waiting is important as PSA screening has become more commonplace and there are an increasing number of patients diagnosed with prostate cancer who will likely die from other causes. It is best used in a patient who has a limited life expectancy from comorbid conditions (eg. severe COPD, heart failure) and is expected to live less than 10 years. Patients with a diagnosis of prostate cancer and life expectancy > 10 years have a significant death rate from the disease and should be offered therapy. This has been shown in a randomised Scandinavian study where watchful waiting patient had an increased rate of death compared to radical prostatectomy at 10 years.
Summary
Watchful waiting is used for patients with significant comorbidities and limited life expectancy. It avoids side effects of treatment in a patient population that is unlikely to develop problems from their disease.