(Exodus 23:3) commands: "Do not honor a poor man in his quarrel." The Mekhilta asks why this verse is needed when (Leviticus 19:15) already says: "You shall not favor a poor man and you shall not honor a great one." That verse covers both extremes — do not favor the poor, do not honor the rich.
From the Leviticus verse alone, we know two things: do not favor the poor and do not honor the great. But we do not know that these biases are interchangeable — that you must also avoid honoring the poor and favoring the great.
The Exodus verse fills this gap. "Do not honor a poor man" — this introduces the concept of honoring (not just favoring) a poor person in a legal proceeding. Now both "favor" and "honor" apply to both the poor and the rich. The two prohibitions cross-pollinate.
The practical application is that a judge must be completely impartial. He cannot favor the poor out of sympathy. He cannot honor the rich out of deference. He cannot honor the poor out of pity. He cannot favor the rich out of calculation. Every combination of bias is blocked. The Mekhilta reads the two verses together to construct a comprehensive firewall against judicial partiality, ensuring that neither wealth nor poverty, neither favor nor honor, can tilt the scales of justice in any direction.