The Reason Being?

I have this thing for Grey's Anatomy. I love it! It's the first medical series I get hooked to. The characters, their lives, their crap, the surgeries and all that surgical info!! It's amazing. To someone with absolutely no medical background, and no interest whatsoever,it's facsinating in a way! Doctors are human too. There's this other side to them aside from them diagnosing your condition, prescribing medicine and telling you to really take better care of your own health.
They have their ups and downs. They think like us! and, here it comes .... they save lives.

So I was watching an episode from Season 3 (a rerun, e7em) and there was a part that just really got to me. It got me seriously thinking.
There's Bailey and Alex operating on a nine-year-old kid, who believes she's a superhero because she doesn't feel pain. She has like a sky rocket tolerance threshold for pain!! She does not know pain. She keeps telling kids to beat her up and punch her in the stomach, because she can really take a beating and it wont hurt her one bit! Not one. No pain at all.
Turns out tho it's internal bleeding from a fancy chromosomal disorder that causes her not to feel hurt, even when she is.
While Bailey is fixing her up, she says " This girl should be put on a poster somewhere to remind us pain is there for a reason"

I just found myself asking... and the reason being?