How will my preemie benefit from a blood transfusion?

Improves oxygenation. Increasing the number of rbcs will increase blood volume which helps to treat hypotension or shock from blood loss. It also improves oxygen in the blood which will decrease the strain on the heart from anemia and also may decrease various signs of anemia like apnea, slow or fast heart rate, poor weight gain, fatigue during feeding etc.
A lot. It will definitively help with anemia of prematurity; it might decrease frequency of episodes apnea and desaturation; it will boost an immune system; it indirectly will improve overall infant status.