The rest of Exodus 16 deals with the institution of a pre-Sinai Sabbath and the people’s response to this. I believe that the same theme of first half of the chapter can be easily brought over and extended into the focus on the manna and Sabbath. That is, we still learn here that true saints believe that God alone can satisfy their greatest desires.
The real force of the story is unveiled in the second part of v 25: There the LORD made for them a statute and a rule, and there he tested them. The LORD was leading his people. He was the one who led them to Marah. The LORD was the one who, in their desperate need for water, brought them first to the bitter waters. But v 25 is the divine perspective on these earthly affairs. Marah was a test. The Lord was trying his people to see if they would trust him. Thus the lesson from God’s Word is, in times of great difficulty, we must trust the Lord.