Pearl is a necessary symbol because, by being unique and mysterious, she gives some essence of female sophistication, and even a supernatural purpose, to Hester's affair. This means that Hester did not just have an affair because she is a lose woman, or a wayward. There might be more than just carnal motivation behind the union of two souls such as Hester's and Dimmesdale's. She also gives purpose life; to existing, even if it is in the form of an outcast.
Pearl is given unique and beautiful traits that far outweigh the ugly environment that forces her odd behavior. We know that she is beautiful, that the colors in her clothing seem more brilliant and shiny, that her behavior is primal, and instinctive. All of these things build up an image of Pearl that detours completely from the horrid image of the demon-child that the villagers aimed to create.
Therefore, the use of Pearl as a supernatural symbol colors the actions of Hester and provides a much needed relief from the ugly and gritty image of Hester that the villagers built. Moreover, the fact that Pearl is an innocent soul born out of what is perceived to be a sinful act gives Pearl's life dyadic meaning; life is not as matter-of-fact as the villagers thing; there is a complexity in existing and it is our actions, and not our circumstances, what make us "good", or "evil". Hence, Hawthorne uses both the physical and the intangible to add dimension to the life of Pearl, and a significance of purpose to the sad life of Hester Prynne.