The Heidelberg Catechism is very helpful to us in explaining the purpose(s) of the Lord’s Supper. Q.75 says:
“Q. How does the Lord’s Supper remind you and assure you that you share in Christ’s one sacrifice on the cross and in all his gifts?
A. In this way: Christ has commanded me and all believers to eat this broken bread and to drink this cup. With this command he gave this promise: First, as surely as I see with my eyes the bread of the Lord broken for me and the cup given to me, so surely his body was offered and broken for me and his blood poured out for me on the cross. Second, as surely as I receive from the hand of him who serves and taste with my mouth the bread and cup of the Lord, given me as sure signs of Christ’s body and blood, so surely he nourishes and refreshes my soul for eternal life with his crucified body and poured-out blood.”
In other words, as a sign of the gospel, the Lord’s Supper reminds us of our salvation in Jesus Christ by grace alone through faith alone.
And as a seal of the gospel, the bread and cup strengthen us in the assurance of our salvation in Christ and make us grow in grace.
So we need the preaching of the Word, and we also need the Sacrament of the Lord’s Supper. Why? Because we are sinners who are tempted to do one of two things: 1. Rest in and rely on our own righteousness, and 2. to despair of our salvation because of our sin.
So we need the bread and the cup to remind us of Christ’s death on our behalf. We need it to shatter any confidence that we might have in our own good works, so that we might once again receive and rest upon Christ alone and his righteousness for our salvation. And we need it to strengthen our faith and assurance in that grace that we have through Christ alone.