Our speaker comes from the very world he's talking about. He knows Ma Rainey, he knows the blues, and most importantly, he speaks the language of black southerners. But if that's all he was, he'd be just another face in the crowd, like the guy who speaks to him in section 4.
The speaker's got an agenda. He wants to tell us, his readers, about all these people, like the man in section 4, because he believes that they, and their love for Ma Rainey, are important. Why else would he spend 52 lines talking about them?
In other words, he's a bit of a folklorist—much like Sterling Brown himself. He finds value in examining the lives of these often-overlooked people, and he wants his readers to understand them the way he does—hence the whole dialect thing.
If he were the fancy, elbow-patch-sporting folklorist of the university set, we might get a poem in perfect meter or, heck, even an academic dissertation. But this guy's much more of the people than about them. He's all about authenticity.
How can we tell? Well, just look at the choices Brown has made in the poem: