Why were the Sophists important philosophically?
The Sophists do not have an august reputation, and their successors in ancient times, particularly Plato, had little praise for their contributions to philosophy. However, that assessment may not be altogether fair. Unlike the Pre-Socratics, who concentrated on the natural, non-human world, the Sophists were interested in human nature and human affairs. The Sophists were the first humanists in Western philosophy. We should also keep in mind that much of their thought was opposed to the timeless wisdom prized by Plato, and much of how they were characterized comes from Plato.
The Sophists were public intellectuals who popularized existing knowledge and wisdom, with some original modification. The subjects they addressed included: grammar, theory of language, ethics, political philosophy and doctrines, religion, ideas about the gods, human nature and the origins of humankind, literary criticism, mathematics, and last but not least, speculations about the natural world that had been developed by the Pre-Socratics.