Joshua has just assumed leadership of the wandering Israelites from Moses and they were headed for the promised land. God promised Joshua that He would be with him just as He was had been with Moses and God demonstrated this by miraculously drying up the Jordan River in the middle just as He had done with the Dead Sea. God believed that this miraculous event was worth remembering and He wanted His people to teach to future generations about it, therefore He told Joshua to build a stone memorial.

When God wants to memorialize something, he uses stones, not statues. Joshua 4:1-24