Christ pointed to a piece of bread and said “eat it, this is my body” (in all synoptic gospels). When challenged on whether he meant it in the literal sense, He made it clear that indeed, “unless you gnaw on it, you will not have eternal life” in the Gospel of St. John. [...] After His resurrection, moreover, He performed another miracle: He appeared to the disciples in Emmaus in such a way that they only recognized Him in the bread. St. Paul also taught the Corinthians that they should “discern” the body of Christ in the Communion. This describes what is known as Real Presence of Christ in the Eucharist. All we do at this point is take the scripture, the words of Christ, literally. I am aware of other interpretations, — that He meant it in some nonliteral sense, but they do not convince me.
The Real Presence is the fundamental Catholic doctrine. It is also the Orthodox belief, and Martin Luther’s belief, and I can point you to Christian writings from the 1st Century that describe the same doctrine, so it is also the historical doctrine since the beginning of Christianity.
The question arises, — How can bread (or wine), which our senses continue to perceive as bread, become body, blood, soul and divinity of Christ? What is the mechanism of the Real Presence?
The answer to this could not be given till the synthesis of classic aristotelian philosophy and Christianity occurred in the Middle Ages. Prior to that, the answer was — We don’t know, just like we don’t know how Jesus walked on water or fed the thousands with five loaves and two fish. It is a miracle and a mystery. This non-explanation remains the Orthodox teaching to this day, and we, Catholics, are fine with it. It is sufficient to maintain your catholicity if you simply say “I don’t know how the Real Presence happens”. Nevertheless, the inquiring mind of St. Thomas Aquinas gave the answer, and his answer is Transubstantiation. What it means, he taught, is that everything has “substance” and “appearance” (the techical term is “accidents”). Usually when the substance changes, the appearance changes also. For example, when a log becomes fire ir looks different than a cold log. But still the substance of the log is not what enters our eyes and nostrils, the accidents of the log do. So, in principle, it is possible there to be a discrepancy between substance and appearance. Such is the case with the Eucharistic elements: the substance of the bread changes over to Christ, — the bread trans-substantiates, — but the accidents remain as before, the appearance and taste of bread.
It is similar to how the disciples are with real Jesus in Emmaus, but they see a stranger and not Jesus, till the end of their conversation.
Luther offered another explanation for the Real Presence, Consubstantiation, of which you need to ask a Lutheran.
You also asked what is the need for it. The simple answer is, it is a commandment of Christ to do it, so priests continue to do it in His name. Christ also explained what benefit it is to us, when He sad that unless we eat the Eucharist, we will not have eternal life.
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Yeah obviously the bread was NOT himself.. I guess Jesus assumed those present had enough sense to know that.. and that they could figure out the metaphor.. By the way some of them were offended and Could NOT or Would Not figure out the metaphor.. and walked with him no longer... Which could have been the purpose of the metaphor.. Jesus maybe figured NOBODY would be stupid enough to see the metaphor as literal.. Wonder if Jesus laughs at (privately) RCC and EO followers.. and those that see many of his other metaphors as literal..