Malbim: It is to refute those who deny the miracle, and say that the river was dry at the time, or Yehoshua knew when is high tide and low tide. The Torah ¹ says that it was on Nisan 10, when the Yarden is normally full, and the place is recognized via the rocks. There the river never dries, nor does it rise or fall (high tide and low tide) due to the moon. Also, the miracle began the moment that the Kohanim's feet hit the water and ended when they rested on the bank. You cannot say that Yehoshua coordinated when they ascend because he knew when the river will get high again. They ascended once the people finished passing!