In 2008 I had open-heart surgery to repair a prolapsed mitral valve. I had prepared for the surgery by having a couple units of my own blood stored for an autologous transfusion, should any transfusion be neccessarry. NO blood at all was used for this surgery.
A few months after this I needed to have a meningioma removed from inside my skull. This was a sudden need with no time to prepare. Again, NO blood was needed during the surgery, which required peeling part of my scalp down to expose my skull, drilling a hole about 3/4 inch in diameter through my skull and cutting away the meningioma that was sitting on my brain like a frog on a log.
The primary surgical need of blood today is to offset hypovolemic shock, which is extreme blood loss as a result of an accident most of the time.
I'm not an expert on blood fractions, but my guess would be that they would be used most often to assist with the treatment of hereditary blood deficiencies or some kinds of poisons.