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 which 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.