Wright begins to use a series of parenthetical remarks, a digression from the story in chapter 15. To what could you compare this technique?
These parenthetical interjections are a modernist technique that interrupt the linear narrative. In other words, the modernist story does not go straight from point A to point B. You are right, they are a digression and in a way delineate a modern concept of time and memory. In 19th century texts we find the present and the past inevitably separate, but in modernist texts the two often collapse or collide. This technique of parenthetical insertion begins with Virgina Woolf and T.S. Eliot, but within American circles we find it in Faulkner's Absalom, Absalom for instance.
Scholarly work quite often interprets such interjections as expressions of memories and desires, an avenue I regard as quite appropriate (check out the book Modernism, Memory, and Desire by Gabrielle McIntyre). This interpretation holds up for Wright's narrator/biographer.
Take a look at chapter fifteen and you will find him "forging in the depths of his mind" , "contemplating", "memorizing" etc. over things past and present, trying to establish a connection that will give him some direction of thought in regard to the race problem that is on his mind. He actually uses the terms "dreams and desires" as well.
Further, the modernist text plays with form quite a bit and often does away with conventions. The formal narrator is often absent or becomes fuzzy at times. What the open ended parentheses in the text mimic is a stream of thoughts from the narrator's memory. They are open ended insertions, hence the open parentheses, or, if you want to look at it from another angle, multiple beginnings and one closing.