“That all the people of the earth might know the hand of YHWH, that it is mighty, that they might fear YHWH your God for ever.”
These two great events were not just a witness to the children of Israel but to the whole world far and wide. They too would be made to recognise the great power of YHWH, and learn to ‘fear' Him (compare Deuteronomy 28:10), especially when One Who was greater than all would one day come up out of that Jordan to become a witness to and sacrifice for that world.
For the fear of YHWH compare Deuteronomy 6:2; Deuteronomy 6:13; Deuteronomy 4:10; Deuteronomy 5:29. Joshua was steeped in the ancient traditions and especially in Moses' teaching in Deuteronomy. It signified reverent awe. We too must remember that like them we must love God and fear Him. The fear of the Lord is the beginning of wisdom. He is not to be treated lightly.