The question guiding this post is this: what significance do Dorothy's "friends" have to the story or the story's thematic ideas?
Each of Dorothy's friends is seeking something from the Wizard of Oz. If you've seen the film at all, you know this. The Scarecrow is seeking brains, the "Cowardly" Lion is seeking courage and the Tin Woodsman is seeking a heart. We know that to find these things, they don't have to go any farther than "their own backyard," but these characters are clearly not self-aware.
As the foursome are traveling to the the Emerald City, they come upon a large ditch. The only one who can actually cross the ditch, after some deliberating, is the lion. This, the lion comes up with after looking at the ditch and calculating how far he can jump in his head. His declaration is the epitome of cowardly behavior. Yes, I'm such a coward that I can jump this large ditch. And to boot, the scarecrow adds that each of the members of the party can ride across on his back. So much for not having any brains.
Then they walk some more and come upon a larger ravine. The lion can't jump this one. Oh, what are they going to do. No, the scarecrow has another idea. Why doesn't the tin man cut down one of the large trees so that it falls across the ravine. Then they can all just walk across. No brains my ass. And they're being followed by these guys who have the body of a bear and the head of a tiger. Our friend the coward staves them off for a moment by growling loudly at them, then scurrying across the log. The brainless one tells the tin man to cut the tree so they fall in the ravine.
Moral more than thematic idea: Being self-aware keeps you from going out of your way to search for something you already have. Then one can argue that the purpose of Dorothy's friends is to show her that she already possesses all the knowledge she needs to accomplish her goal, she just has to look inward to find it.