Charles Ellicott Commentary Exodus 17:15

"And Moses built an altar, and called the name of it Jehovah-nissi;" — Exodus 17:15 (ASV)

Moses built an altar. —Primarily, no doubt, to sacrifice thank-offerings upon it, as an acknowledgment of the Divine mercy in giving Israel the victory. But secondarily as a memorial—a monument to commemorate Israel’s triumph.

And called the name of it Jehovah-nissi. —Jacob had named an altar El-Elohe-Israel (Genesis 33:20); but otherwise we do not find altars given special names. When an altar was built as a memorial, the purpose would be helped by a name, which would tend to keep the event commemorated in remembrance. Jehovah-nissi—“the Lord is my banner”—would tell to all who heard the word that here there had been a struggle, and that a people who worshipped Jehovah had been victorious. It is not clear that there is any reference to the rod of God (Exodus 17:9) as in any sense the “banner” under which Israel had fought. The banner is Jehovah Himself, under whose protection Israel had fought and conquered.