For Catholics the last supper was the summit of Jesus’ ministry on earth and is so important now because a mystery occurs every time that it is performed; transubstantiation. This is the turning of the bread into the literal body and blood of Christ. This is not to say that it changes form and that you would be able tell the difference with your senses but the reality of it has changed.
So no matter what Christians believe happens at the Eucharist and how they “dress it up” (or indeed dress it down) it is a central point of reference for all Christians and, along with Baptism, is something that you can almost guarantee that a practising Christian has participated in.
If you are a practising Christian comment below to give your insight on the importance of the Eucharist.