For someone in their 20s or 30s who is in good health, are annual physicals necessary? If so, why? If not, why not?

Low yield but useful. While the yield is low for finding significant pathology on routine physical examination in young, healthy adults in this age range, this is the age where early detection of breast cancer, heart disease or other potentially sinister maladies may save lives. If you are the individual who has a problem detected early, you will definitely think that the annual physical was necessary.