Why do we need the Eucharist for the forgiveness of sins if we have already been justified by faith?

Answer: Consider the following analogy:

A child sins against his parents, and feels guilty afterward. Has his sin, or the resulting guilt, caused him not to be children of his parents? If his parents are godly, the answer is no.

However, there is a sense in which the relationship has been "broken" -- not actually, but affectively. There needs to be reconciliation -- not so that this child remains a child to his parents and vice versa,

but so that the true sense of this relationship of care, love, and respect be restored.

The child may even indeed come under the discipline of his parents. And he may come later, asking for forgiveness.

When his parents grant him their forgiveness, if they are godly, it is not as though their child's contrition changed the relationship between them; it's rather as if their relationship effected that child's contrition (because he knows they will forgive him).

If these are godly parents, they have actually already forgiven him before he ever asked, and were not avenging themselves by disciplining him but rather seeking to draw him to repentance.

When they express their forgiveness to their child, it is so that he may be assured that they truly love him and therefore experientially be forgiven.

He was already forgiven in the proper sense before they spoke, but in order that he may know it more fully, his parents declare it to him with their words, and a warm embrace.

The eucharist is this declaration of forgiveness of sins from God our heavenly Father to us his earthly children.