Good question, Mrs. Baran!
I feel that the reason O'Brien goes into such detail to describe Mary
Anne after her time out on ambush was to emphasize the drastic change
in her character and appearance. When Mary Anne first came she was
described as a bubbly, friendly, happy-go-lucky type of person. She
could find the good in almost any situation, and the fact that she was
at a war camp didn't seem to even phase her. Little by little though,
she gets sucked into the war, she sneaks out to go on ambush and when
she comes back, the effects of the war are extremely evident. She's
completely changed, but she doesn't hate it. "I get scared sometimes
-- lots of times -- but it's not bad. You know? I feel close to
myself." (O'Brien 106). Maybe O'Brien was trying to show us how it
feels to be at war. He uses a woman so that she's more relatable to
those who haven't been at war. By describing her as someone who looks
like some sort of tribal warrior after she comes back from being on
ambush, he tries to show us how the men feel after being at war. It's
like this unknown side of them has been revealed: a warrior-like,
gruesome side. But, it's not a bad side, necessarily. It's a side
that's necessary for them to have in order to be able to go to war -
to fight and to kill.