Rashi asks if it’s possible that Moshe spent all day every day doing nothing but judging matters for the people. The Talmud (Shabbos 10a) wonders the same thing: when did Moshe have time to study Torah? The answer given there is that this verse teaches us that one who judges a case in complete truthfulness, even for a brief time, is considered G-d’s partner in creating and sustaining the world. (The “morning” and “evening” in our verse serve as allusions to the mornings and evenings in the Creation account in parshas Bereishis.)