11 And the LORD said to Moses, "How long will this people despise me? And how long will they not believe in me, in spite of all the signs that I have done among them?
12 I will strike them with the pestilence and disinherit them, and I will make of you a nation greater and mightier than they."
God is understandably upset at Israel's continued unbelief. He tells Moses that he will destroy Israel and then take Moses to make a nation of him instead. What an offer this is! Yet Moses does not want this and intercedes for the people, convincing God to stick with Israel.
On the surface, it seems strange that God would allow Moses to convince him to change his mind. After all, isn't God aware of the past, present and future; and isn't he able to do all that he wishes?
However, we must remember that God does things for a reason, and if this conversation (or demonstration) between Moses and God didn't exist, recorded in scripture, we would not know of God's great mercy in forgiving those deserving of such incredible punishment for their actions, including even all Christians (who will continue to act in unbelief from time to time until the end).