Because it is food that give life for our souls. Just as our physical bodies need food to be strong and live, so too are our souls; they need spiritual food and Jesus Body and Blood is the perfect nutrition.
“Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you; he who eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day. For my flesh is food indeed, and my blood is drink indeed. He who eats my flesh and drinks my blood* abides in me, and I in him**. As the living Father sent me, and I live because of the Father,** so he who eats me will live because of me**. This is the bread which came down from heaven, not such as the fathers ate and died; he who eats this bread will live for ever.” Jn 6:53-58*