We do it in remembrance of Jesus' sacrifice for all humanity when He died in our place on the cross so that we would never be separated from Him and have eternal life. "For God so loved the world that He gave His one and only Son... (John 3:16). He literally became the Passover lamb .. the Lamb of God. Scripture says some struggled with this concept and walked away. It's one of many mysteries of God. Jesus said for us to remember Him in this way. That's why the church has communion. But the ones who follow the enemy of our souls perverted this and use a sick "copy" of it in their unholy rituals.