In Biblical typology, the authors of the New Testament often relate things of the NT in light of the Old Testament. This is called typology. You see Paul speak of this in Romans 5 when he identifies Jesus as the superior antitype of Adam. You see the Pauline tradition in Hebrews (ch 8) speak of this when he compares the sacrifices of the OT to the corresponding superior sacrifice of Christ. You also see Jesus speak of this earlier in the John 6 discourse when he spoke of the bread, the manna, that fell from heaven. One consistent characteristic in the order of typology is that the New Testament antitypes are superior to their Old Testament types. Jesus is superior to Adam. Christ's sacrifice is superior to the OT sacrifices. And the Bread of Life in the NT is superior to the manna that fell from heaven.
Catholics believe the Bread of Life, of which Christ spoke in John 6, is the Eucharist, the true body and blood of Christ in sacrament (cf. CCC#1374). Some faith traditions believe that the Eucharist is symbolic-only.1 They believe the bread is ordinary bread, and participating in the Eucharist is a memorial in the sense of "calling to memory" Christ's sacrifice (not in the sense of the re-presentation of the event according to the Jewish understanding of anamnesis2).
Now, if we apply a "symbolic-only" understanding to John 6, we cause a fatal problem in the order of Biblical typology. The NT Bread suddenly becomes inferior to the OT manna. After all, the OT manna was 1) of supernatural origin and 2) of benefit for physical life. When we insist the Bread in John 6 is symbolic-only, we make it inferior to the OT manna because we say its origin is less-than-supernatural, while denying that it is of benefit for eternal life.
It is Christ himself who made the typological comparison between the Bread of Life and the OT manna in John 6:49-51. And therefore, the symbol-only interpretation must be rejected, among other reasons, on the grounds that it violates the superior nature of NT antitypes over their OT types.
1For example, the Southern Baptist Convention in 2000 endorsed the following: "The Lord's Supper is a symbolic act of obedience whereby members of the church, through partaking of the bread and the fruit of the vine, memorialize the death of the Redeemer and anticipate His second coming." In Catholic theology, the Eucharist does have symbolic attributes, but not only symbolic. For examples see Council of Trent 13.3; Pope Paul VI, Mysterium Fidei, et al.
2See for example, Rabbi Dr. Stuart Dauermann's explanation of the Jewish idea of anamnesis in the article Seeds, Weeds, and Walking the High Wire: The Role of the Remnant - Embodying Israel’s Destiny. He writes in one example: "The holy past is no mere collection of data to be recalled, but a continuing reality to be honored or desecrated."