Moses carried God's message to the people of Israel. He delivered the divine offer: accept the Torah, become a kingdom of priests and a holy nation. The people responded with unanimous agreement. Then Moses did something that the Mekhilta finds remarkable — he went back and reported their answer to God.
"And Moses returned the words of the people to the Lord" (Exodus 19:8). The Mekhilta pauses on this verse and asks a question that seems almost impertinent: was this really necessary? God is omniscient. He already knew what the Israelites would say before Moses even opened his mouth. Every thought, every murmur, every heart's intention lay bare before the Creator. Why would Moses bother reporting back?
The answer has nothing to do with information transfer. The Torah is teaching derech eretz — proper conduct, basic etiquette, the way a person ought to behave. When someone sends you on a mission, you return with a report. Even if the sender already knows the outcome. Even if the answer was obvious from the start. The act of reporting back honors the relationship between the messenger and the one who sent him.
Moses understood this. "Though He knows," Moses reasoned, "I shall return an answer to my sender." The greatest prophet in Israel's history, speaking with the omniscient Creator of the universe, still observed the basic courtesy of completing his errand with a proper report. If Moses did not consider himself above simple etiquette, the Mekhilta implies, neither should anyone else.