Your question is similar to asking why there is a distinction between ?fruit? and ?apples.? ?Fruit? is a category, and ?apples? is just one item that is in that category.
?Emergency? is a category that includes both ?Life Safety? and ?Critical.? So the question should be, ?why is there a need to distinguish between ?Life Safety? and ?Critical??
That answer lies in the definition of those two terms, a definition found in 517.2 (not in 100). The ?Critical Branch? is all about being able to provide patient care, in the event of a loss of the utility power source. The ?Life Safety Branch? is the more important of the two. It is all about keeping the patients (and the staff) alive and safe, in the event of a loss of the utility power source.