For being a little kid, Phoebe is one sharp cookie, and she acts as a guide for Holden in a variety of ways. First, she asks him important and incisive questions (as in, "You don't like anything. Name one thing you like a lot"). She helps him in little ways, sure, by covering for him when their mom comes home, lending him money, and putting the red hunting hat on his head when it starts raining.
But more important than these gestures are what they represent; Phoebe understands Holden in a nuanced and sophisticated way, and though she's younger than he is, she's incredibly adept at guiding him through his toughest times (like when she puts her arm around his shoulder while he cries). When she brings a suitcase because she wants to run away with Holden, it's possible that she wants to take care of him as much as she wants to run away.
She also teaches her brother a lesson, if unintentionally; when he watches her reach for the gold ring on the carousel, he's able to draw a conclusion about the inevitability of growing up. So Phoebe Caulfield: the greatest ten-year-old mentor ever. Also, check out "Booker's Seven Basic Plots Analysis" to see that Phoebe also fits the role of the "heroine or small child" that "rescues" the "hero" from a "dark power" at the end of the story.