I used to think God was pretty harsh on Moses. After he obeyed God and tapped the rock from which gushed forth a fountain of water to sustain the chosen people he was denied entry into the Promised Land. You see, he tapped the rock twice. Why did that get him on the wrong side of the Lord?
When God does something, even through a sinful and weak human instrument like Moses, God does that thing perfectly and therefore only need do it once. Moses was a superb "front office" manager as God called him to be for his time and place in salvation history but failed on that one occasion to represent corporate headquarters well even though the end result was the same as if he had struck the rock only once.
Jesus Christ offered Himself perfectly once and for all in the water and blood which gushed forth from His side on the Cross. But the work of providing His gift to every human person must continue until the end of the world. The ordained priest is a new type of Moses as "alter Christus" and taps the rock of the altar, symbol of Christ, every time he celebrates the holy Sacrifice of the Mass. This is one reason why the Church in her noble custom has always preferred a stone, or marble, altar to one made of less permanent material such as wood.
In the Blessed Sacrament, which the priest consecrates in each perfect self-offering of the Rock of our salvation, gushes forth once again His Flesh and Blood which provide the perfect food for His pilgrim people here and now.