Ben, who has achieved great success in life and is constantly held up as an example by their father, Willie, is a symbol of Biff's internalized sense of inadequacy. He feels like he can never live up to Ben's accomplishments and is constantly trying to prove himself. The mock fight acts as a way for Biff to release some of his pent-up frustration and express his desire to be recognized in his own right.
At the same time, the fight also highlights Biff's dependency on Ben. Despite their differences and Biff's resentment towards him, he still looks up to Ben and seeks his approval. The mock fight suggests that Biff is caught in a cycle of trying to assert his independence while also craving the validation of his older brother.
Ultimately, the mock fight between Ben and Biff symbolizes Biff's unresolved internal conflicts and his search for self-identity. It is a manifestation of the pressure Biff feels to succeed, the weight of his father's expectations, and his own longing for recognition and approval.