The Sfas Emes asks a question about the second set of luchos. His answer is even better than the question, IMHO. I read this over ten years ago and still come back to it several times during any given year. Have a great Shabbos Kodesh!

Why did Hashem require Moshe to spend a second forty-day period on Mount Sinai simply to receive the Second Tablets, when he had surely mastered the entire Torah during his first stay?

During Moshe’s first stay on the mountain, he was given preparation to teach the Torah to a nation of tzaddikim, as befitted the nation’s status at that time. Now that the people had sinned and struggled to repent, he had to be equipped iwht an entirely different methodology suitable to their changed condition as pentitents. The special needs of the baal teshuvah in attaining Torah knowledge are alluded to in Berachos 34b, In the place [i.e. the apprach to learning Torah] where the baalei teshuva stand, even the completely righteous are unable to stand.

Let us consider further the permanent impact left by the Second Tablets on our people. Hashem sealed His relationship with us in the form of a covenant, as the Torah states (Shemos 34:10): Behold! I seal a covenant before your entire people. The Second Tablets, which were made of stone hewn by Moshe, typified covenants in which both parties commit themselves to contribute to the common good.

Although human involvement in the Second Tablet might seem to detract form their importance, actually they speak to the condition of the contemporary Jew far better than the First Tablets, even though they were made entirely by Hashem. So removed is our generation from the experience of Mount Sinai that we cannot possibly aspire to reach the lofty levels proffered by the First Tablets. The Second Tablets, however, precisely because they embody human participation, continue to act as the backbone of our national existence until this very day. (From DAYS OF AWE: SFAS EMES pages 26-27)