Our Lord Himself gives the reason, equating - though in an infinitely higher manner - the manna with which the Hebrews were fed during the forty years in the desert to His Body which will be given for our salvation. The Hebrews had to eat the manna to survive, we have to eat the Bread from Heaven for life eternal in union with Him.
John 6:41-51:" . . .The Jews were complaining to each other about him, because he had said, because Jesus said, “I am the living bread that comes down from heaven.” They were saying, "Surely this is Jesus son of Joseph, who father and mother we know. How can he now say, “I have come down from heaven?” Jesus said in reply to them, "Stop complaining to each other.
“No one can come to me unless drawn by the Father who sent me, and I will raise that person up on the last day. It is written in the prophets: They will be taught by God everyone who has listened to the Father, and learnt from him, comes to me. Not that anybody has seen the Father, except him who has his being from God: he has seen the Father. In all truth I tell you, everyone who believes has eternal life, I am the bread of life. Your fathers ate manna in the desert and they are dead; but this is the bread which comes down from heaven, so that a person may it and not die. I am the living bread wiich has come down from heaven. Anyone who eats this bread will live for ever; and the bread that I shall give is my flesh, for the life of the world.” (New Jerusalem Bible).