The reason is that the bread and wine can only benefit us if we receive them by faith. They are nothing in themselves, only signs of something else. In fact those who eat and drink in an unworthy manner, says Paul, eat and drink judgment on themselves.
Faith in Christ is needed because the sacrament points us to Christ’s death. It tells us that Christ died for us. He shed his blood, gave his life, so that we could be forgiven, so that we could be reconciled to God.
When we take the bread and drink the wine we remember the promise of the gospel. We are being offered life, forgiveness, cleansing on account of Christ’s death. We can take and eat and not believe this – at best an act completely useless, at worst bringing God’s judgement on us.
Or we can eat and drink with heartfelt thankfulness that Christ has died for us.
The words of administration (in the Prayer Book) help us understand this. Before the reformation some versions of the Mass used the words “The body of the Lord Jesus Christ keep thy body and soul unto everlasting life.” These words could be understood to refer to the bread itself as being the actual body of Christ (as the doctrine of transubstantiation taught).
In the first English version of the Communion service in 1548-49 this was changed to “The body of our Lord Jesus Christ which was given for thee...”. This made it clear that the focus was on the death of Jesus, not the bread.
So when we take the bread (and wine) we need to believe that Christ’s body was given for us – and trust his death to keep us to everlasting life.
Dale