Mrs. Pearce provides a way into the drama and its characters for the audience. She gives us a little glimpse beneath the surface of other characters in the play, especially Henry Higgins. It is through Mrs. Pearce that the contradictions of Higgins's complex personality are revealed. As well as calling him out for his breathtaking arrogance and rudeness, Mrs. Pearce forces Higgins to confront his singular lack of self-awareness; he wants to change Eliza, but perhaps, she subtly insinuates, he should start with himself.
As with most of Shaw's audience members, Mrs. Pearce is middle-class. This is important as it gives the audience a voice, a means of articulating the generally negative feelings they hold towards Higgins and the corresponding sympathy they feel for Eliza. But as Eliza starts to grow in poise, confidence, and self-respect, there is no longer any further need for a character on stage to express our sympathy for her. It is notable, then, that Mrs. Pearce's role in the play diminishes accordingly, as now Eliza, neither the humble Cockney flower-girl she once was, nor quite the lady of quality of Higgins's imagination, has effectively become a younger version of Mrs. Pearce: respectable, middle-class, and more than capable of standing up for herself.