Why is the scene when Danforth questions Elizabeth about John's faithfulness so important to the plot of the play ? (What Elizabeth says and what the significance that has.)

Proctor is a tortured man. His unfaithfulness to his wife eats at his moral core like a cancer. Danforth questions Proctor's fidelity to his wife to bring disrepute to his character and hence his testimony in court. Elizabeth lies for the first time in her life to save her husband. Of course Proctor had already confessed to the adultery so her lie backfires.