The whole theme of Langston Hughes's "Mother to Son" is that it is necessary to keep working hard, to keep climbing, in order to get ahead. The diction (choice of words) and imagery of the poem reinforce that.
The central image of the poem is the stairway. This image encourages us to think about the upward progress that the mother wants her son to make. This image, with its talk of splinters, and twists and darkness reinforces the theme that one should keep working upwards, no matter the obstacles.
The choice of words is, I believe, meant to illustrate how the mother has not herself "made it." She uses "dialect" much of the time and sounds uneducated. But the point is that she has worked hard and given her son a base and he needs to keep climbing from where she has gotten him.