The importance of remembering...

At the end of Exodus chapter 16, we read about how the people of Israel preserve some manna in a container as a memorial of what God had done for them. Manna wouldn't be around for ever and so they wanted future generations to be able to see and hear of how God had provided for His people during their wilderness journey. Then, at the end of Exodus chapter 17, God instructs Moses to write on a scroll about the amazing victory they had just experienced over an enemy, the Amalekites. The people, with their grumbling against the Lord, had already demonstrated how quickly they forgot what He had done for them. These two actions were ways of helping them remember better in the future:
a) remembering His provision for them - God's goodness
b) remembering the victory He gave them - God's greatness

We would do well to find our own ways of reminding ourselves of God's goodness and greatness in our lives. Nowadays it might involve taking a photo or video, writing a journal or book. Churches might put up a plaque or a banner; individuals and families might hold on to a physical item that represents in some way what God did for them.

In this instance, at the end of Exodus 17, Moses also did something that held great meaning for Old Testament believers: he built an altar where God could be worshipped and, interestingly, he named it Yahweh-Nissi, which means, "the Lord is my banner." A banner proclaims our allegiance - like when a political group carries one during a march or football fans wave one during a big game. "The Lord is my banner" is simply a way of saying, "I belong to Him, and I'm not going to forget the things He has done for me."