In Moses' case, there are two things to consider. One, Moses as a mediator prefigured Jesus. Jehovah was using Moses to demonstrate how he leaves the judging to his Son, to mediate between God and man.
Two:
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For example, consider Jehovah’s words to Moses after the Israelites had made a golden calf to worship. God said: “I have looked at this people and here it is a stiff-necked people. So now let me be, that my anger may blaze against them and I may exterminate them, and let me make you into a great nation.”—Ex. 32:9, 10.
14 The account goes on to say: “Moses proceeded to soften the face of Jehovah his God and to say: ‘Why, O Jehovah, should your anger blaze against your people whom you brought out of the land of Egypt with great power and with a strong hand? Why should the Egyptians say, “With evil intent he brought them out in order to kill them among the mountains and to exterminate them from the surface of the ground”? Turn from your burning anger and feel regret over the evil against your people. Remember Abraham, Isaac and Israel your servants, to whom you swore by yourself, in that you said to them, “I shall multiply your seed like the stars of the heavens, and all this land that I have designated I shall give to your seed, that they may indeed take possession of it to time indefinite.”’ And Jehovah began to feel regret over the evil that he had spoken of doing to his people.”—Ex. 32:11-14.
15 Did Moses really need to correct Jehovah’s thinking? By no means! Although Jehovah expressed what he was inclined to do, this was not his final judgment. In effect, Jehovah was here testing Moses, just as Jesus later did Philip and the Greek woman. Moses was given an opportunity to express his view. Jehovah had appointed Moses as mediator between Israel and Himself, and Jehovah respected His appointment of Moses to that role. Would Moses succumb to frustration? Would he take this opportunity to encourage Jehovah to forget about Israel and to make a mighty nation from Moses’ own descendants?
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According to some scholars, the Hebrew idiom rendered “let me be” at Exodus 32:10 could be taken as an invitation, a suggestion that Moses would be allowed to intercede, or ‘stand in the gap,’ between Jehovah and the nation. (Ps. 106:23; Ezek. 22:30) Be that as it may, Moses obviously felt comfortable expressing his opinion freely to Jehovah.