Symbolism. Elijah had a huge audience and now everyone was watching him. So he sets about making a demonstration that people will remember. Read 1 Kings 18:31-32.
Elijah took twelve stones according to the number of the tribes of the sons of Jacob, to whom the word of the Lord had come, saying, “Israel shall be your name.” So with the stones he built an altar in the name of the Lord , and he made a trench around the altar, large enough to hold two measures of seed.
1 KINGS 18:31-32 NASB
The kingdom is divided, but Elijah still goes through the steps to help people to see and understand that God is over all of that. The twelve stones for the twelve tribes. He wants to people to remember God and all that he is and has done for them.
Do we remember and recognize all that God has done for us? Do we set up things that will remind us of all that God has done for us in the past? God is the same yesterday, today, and forever and we need to remember all that He is and all that He does.