Given that Uriah was able to obtain Wickfield's signature on documents, it's not completely clear why he ever needed to forge it. In terms of the novel's themes, however, one possible explanation is that Uriah ultimately hoped to entirely usurp Mr. Wickfield's position—not just in the firm, but also as the master of the house and of Agnes. Symbolically, forging Wickfield's signature is a way of taking his place, or even his identity. It also, of course, provides Micawber and Traddles with concrete proof of Uriah's crimes, which leads Mrs. Heep to plead with her son to compromise; ultimately, Mrs. Heep is slightly more sympathetic than her son, if only because she does seem to care for and value her family.