Let's try changing a section to first-person with Roger speaking and see how the change affects our perception of Roger and the conflicts he faces.
She was a large woman with a large purse that had everything in it but hammer and nails. It had a long strap, and she carried it slung across her shoulder. It was about eleven o’clock at night, and she was walking alone, when I ran up behind her and tried to snatch her purse. The strap broke with the single tug I gave it from behind. But MY weight and the weight of the purse combined caused ME to lose MY balance so, instead of taking off full blast as I had hoped, I fell on MY back on the sidewalk, and MY legs flew up.
“If I turn you loose, will you run?” asked the woman.
“Yes’m,” I SAID.
“Then I won’t turn you loose,” said the woman. She did not release ME.
“I’m very sorry, lady, I’m sorry,” I WHISPERED.
The first analytical impression from this change is that the story loses some of its sincerity and pathos [pathos: the quality of evoking a feeling of pity or compassion (Random House Dictionary)].
The most significant thing about this story is the objective comparison between the lady and the boy. It is this external objective, honest view of them both that gives the genuineness to the connection that develops between them when she confides her past to the boy. Without the third-person perspective, the experiences, the confidences, the reactions are subject to subjective interpretation and biased misrepresentation.
In other words, there is always an underlying question of whether a first-person narrator is reliable, authentic, trust-worthy and accurate in their perception of experience [especially since it is very common for suffering, neglected people to misinterpret others' expressions, tones of voice and intentions] and accurate in their account of words and actions.
To apply this to Roger, if Hughes had chose Roger for a first-person narrator, our perception of Roger would be altered in that we would not have confidence in the sincerity of the account; we might suspect Roger of "tweaking" the telling of the story to his advantage and/or to the lady's disadvantage. Our sympathy and pity for his conflicts and for the choices he has made thus far in life would be diminished as our sense of his unreliability made him seem more or less sincere. In short, this story would change with Roger as the first-person narrator and become a dark story fraught with many opportunities to distrust and misunderstand Roger and misinterpret the lady.
As it is, the third-person narrator gives us assurance that this is just exactly the way it happened and that the motives we attribute to a sincere and genuine Roger and an equally sincere and genuine lady are exactly the motives they had. The third-person narrator acts as our narratorial third party source of confirmation of authenticity.