The people didn’t just need a drink. Moses was supposed to give them water that would remind them of their salvation. By doing things his way, Moses obstructed the story God was trying to tell and obscured the gospel in their hearts.
The rock of Meribah was a powerful symbol that would point backward to Egypt, point forward to Christ, and save the people from their thirst—all at the same time. As they drank from the rock, they would remember how God had broken Pharaoh for their deliverance. They would also know that God’s favorite way to work is to bring the richest blessings out of the worst circumstances.
Moses acted according to his old ways. He spoke harsh words to the people and “killed” the rock, and in that moment he was the Old Moses who “struck” the Egyptian and buried him in the sand. He rejected his commission, which was to “speak and deliver.” Instead, he used his old ways, which was to “strike.” Because of this, God did not get the glory of his people remembering his deliverance.