According to those who hold that the bread and wine do not merely represent the body and blood of Christ, but actually are those things, why are those things eaten?
Should they not be buried?
Why are we "eating" the flesh and blood of Christ?
1. Because Jesus commanded us to
In the institution of the Eucharist, Jesus explicitly commands us to eat the bread and drink the wine, saying "This do in remembrance of me." Indeed, as often as we do this, we do "proclaim the Lord's death until He come."
Indeed, John goes even further, recording Jesus in dialogue saying:
52 The Jews then disputed among themselves, saying, “How can this man give us his flesh to eat?” 53 So Jesus said to them, “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. 54 Whoever feeds on my flesh and drinks my blood has eternal life, and I will raise him up on the last day. 55 For my flesh is true food, and my blood is true drink. 56 Whoever feeds on my flesh and drinks my blood abides in me, and I in him. 57 As the living Father sent me, and I live because of the Father, so whoever feeds on me, he also will live because of me. 58 This is the bread that came down from heaven, not like the bread[c] the fathers ate, and died. Whoever feeds on this bread will live forever.” 59 Jesus[d] said these things in the synagogue, as he taught at Capernaum.
The fact that Christians "ate the body of Christ" and "drank his blood" led to the belief in ancient Rome that Christians were cannibals. This was one of the reasons they were persecuted. That said, the point was that Christians were completely identifying with Christ, and not just filling the belly.
2. Because Priests consume the sacrifice
1 Corinthians 5 says:
For even Christ our passover is sacrificed for us: Therefore let us keep the feast.
This indicates that Christ, being the Passover, is the sacrifice. As a "royal priesthood," the pattern would be to handle the sacrifice as was custom. In Dueteronomy 18:1, we read of the Levites:
“The Levitical priests, all the tribe of Levi, shall have no portion or inheritance with Israel. They shall eat the LORD’s food offerings as their inheritance.
Another meaning of eating Christ's flesh regards the Passover. Just before the exodus, the Hebrews were commanded to kill a lamb without blemish (i.e. "perfect") and use the blood to paint their doorposts and lintels so that God's angel would "pass them over" and not strike their firstborn sons dead (Ex 12). They were also commanded to remember this event every year and pass this story on to their children (Ex 13:14).
The Last Supper was a passover meal. At it, Jesus broke the unleavened bread that they were eating and established the eucharist, saying it was his body. There's some symbolism here regarding Jesus being the Passover lamb that saves us from death.
Not being familiar with the teaching of transubstantiation, I can only refer you to Jesus' own words in this regard, as recorded in John 6:63. I'll include those words here and then comment on them.
"'It is the Spirit who gives life: the flesh profits nothing; the words that I have spoken to you are spirit and they are life."
Not only were Jesus' fellow Jews in the synagogue where He uttered the words about the necessity of eating His flesh and drinking His blood (see Affable Geek's block of text in yellow, above) offended, but
"many of His disciples, when they heard this [i.e., the teaching on His being the bread of life, of whose flesh we must partake] said, 'This is a difficult statement; who can listen to it?'" (v.60).
The offense taken by the Jews, and the grumbling evidenced by many of Jesus' disciples were triggered by their failure to realize Jesus was speaking in spiritual, not physical terms. Put differently, Jesus was speaking in metaphorical, not literal terms.
Jesus often used this tactic, if you will, for a number of reasons:
To get people's attention
To make people uncomfortable, to the point where they would clamor for clarity from Jesus
To expose the spiritual blindness and deafness in His critics and the "easy believers" among His disciples (see Matthew 13:13 NIV, where Jesus said,
"'This is why I speak to them in parables: 'Though seeing, they do not see; though hearing, they do not hear or understand'".
Just as Jesus' parables were designed both to offend people and to expose their unbelief, so also was His teaching on the bread of life, with its reference to eating His flesh and drinking His blood.
In conclusion, Jesus and His hearers were on different wavelengths, spiritually speaking, and Jesus used the hyperbole of His words to speak life into their lives. Again,
"'. . . the words I have spoken to you are spirit and are life'" (v.63).