Joshua 7:26 And they raised over him a great heap of stones unto this day. So the Lord turned from his anger. Wherefore the name of that places was called the valley of Achor unto this day.
Since humans do not see God, His Holiness, or His Love, we all need reminders. In order that they might never forget, the children raised up a heap of stones upon the place where Achan and family were stoned and burned for stealing goods that were dedicated to God. When they see the heap they would never forget that the Lord is serious in all His Commands, that His Holiness cannot be polluted, and that although He is slow to anger Sin still offends Him.
After carrying out God’s judgment and setting up a memorial, the Lord turned from His anger toward sin. Actually, God had given many physical signs and symbols of His holiness and love in the sacrifices, rituals, clothes, and utensils, etc. in Leviticus and other Books of Moses. All of this was to serve as reminders: God is holy and that cannot be compromised; God is Love and that cannot be diminished. When you see the heap of stones you will remember.
The Lord has also given His NT people earthly reminders of heavenly truth in Baptism and the Lord’s Supper. These are visible realities of Jesus, who has replaced and fulfilled all the OT reminders of God. NT believers respond by attending a gathering every Sunday to hear the Gospel, to remember Jesus, who took the anger of God upon Himself, and receive His blessing of forgiveness in Communion, and to remember our Baptism into Christ every day. We are reminded that Jesus was judged for our sins and God’s righteous wrath is turned away from us. Christians also build churches and memorials of all kinds in the valley of Achor (which means trouble), that is, in this Vale of Tears where we sojourn.