...is they’re an analogy for what happens when we receive Communion.
Both bread and wine take many individual things—kernels of wheat and
grapes—and combine them in such a way that the individual become truly
“one”. In a similar way when we receive the Eucharist we become one
with Christ and with each other (1 Cor 10:16). As the wheat and
grapes are melded into one “body” so we’re melded into one body with
Christ as the head. When we say we’re in “communion” with each other it
goes far beyond the traditional clique, club, or team, we become
spiritually altered and meshed together.
This is why we must
love our neighbor; they either belong to our body or we desire them to
if they don’t. This is why there’s no such thing as a sin that “only
hurts me”; anything that hurts me hurts the rest of the body (1 Cor
12:26). If one kernel /grape were poisoned the entire loaf/bottle would
be compromised and so it is with us.
This is yet another reason Catholics use bread and wine for Communion.