All throughout the text, Codi makes a huge deal of the orthopedic shoes she and Hallie had to wear. They both hated them, burning the catalogues when they arrived in the mail as kids so that their dad wouldn't be able to buy them. But what's the big deal? Seriously, Codi, you're not the first kid to ever have to wear lame shoes to school.
The big deal is that Codi already feels like an outsider in Grace. The shoes made her feel like a total alien. She ditches them for a pair of gladiator sandals the second she's out of Grace and away from Doc's watchful eye. Yet when she and Uda are clearing out the attic, they find every pair of shoes he ever bought for them stored up in the attic, arranged in order of size.
[Uda] bent over beside me and picked out one of the smallest shoes, cradling it like an orphaned bird.
"Law, he was so careful about you girls and your feet. I remember thinking, Oh, mercy, when those girls get big enough to want heels there's going to be the Devil to pay."
I laughed. "He wasn't just careful. He was obsessed."
Uda looked down at me. "He just wanted awful bad for you kids to be good girls," she said. "It's hard for a man by himself, honey." (22.69-70)
The image of Uda holding the shoe like an orphaned bird is pretty instructive, considering that Codi and Hallie are called "the orphans" by their neighbors. Basically, the shoes are a synecdoche for the Noline girls themselves.
Symbolically, the shoes stand in for Doc's inadequate and misguided care for his daughters. Codi says outright that he almost never touched them unless her was measuring the bones of their feet, yet he kept every shoe. Those two facts say a lot about Doc—his belief in the importance of the bone structure of the foot is a way for him to try and set his girls up for good, happy, and healthy lives—but it takes the place of the kind of touch, affection, and attention that are essential to raising kids.
At the same time, Doc's archiving of every pair of shoes the girls ever wore is a testament to his love for and attention to his daughters, an attention they never actually got to feel as little kids. Shoes, after all, are there to stand between the human body and the physical world. And they're no substitute for fatherly affection.