PFDs keep you floating. Even if you are a good swimmer, a PFD helps you avoid getting too tired to swim. If you should hit your head or lose consciousness, a PFD can make all the difference. PFDs help you avoid hypothermia, which is a life-threatening condition in which your body rapidly looses heat. PFDs in bright colors, or with reflective tape, make you easier to find in the water.
Nobody expects to have an accident.
Even if you are the safest, most competent skipper, you can't predict what other boaters will do. A boat speeding by can create a wake that can easily tip a smaller boat. If you are in the smaller boat, you could find yourself in the water through no fault of your own. Sometimes mechanical things go wrong with your boat. Or bad weather comes up.