It is difficult to link Grace to any specific plot actions definitively as her appearance is so brief.  Some consider her entrance and the seduction scene between her and Willie to act as a foil or at least a contrasting action to the seduction that occurs between Berniece and Lymon.  The first is so rushed, they can't wait for a bed, etc. so they start making out on the couch, get in trouble, get tossed out, etc.  The second, in contrast, is somewhat magical with Lymon changing once he gets dressed in the fancy suit into someone greater than himself.  Along with that is the fact that Berniece gains something from the encounter, perhaps the power to be a real woman as she finally has a man, whereas Willie and Grace's encounter leads to nothing but a forgotten night and some minor conflict.