It shows that God’s miracles protected our ancestors in days of old: it assures us that He will be with us as we move through the day and through history.
There are several views as to where the passage ends.
Some say it goes up to verse 27 of Ex. 15, “For I the Lord am your Healer”. Some say it ends at verse 18, “The Lord will reign for ever and ever”. Others prefer verse 19.
The most common tradition opts for verse 18, which is underscored by repeating the words HaShem yimloch l’olam va’ed.
The justification for this view is not simply that it is a literary crescendo. Rashi and the other commentators tell us that the God whom we worship will eventually be acknowledged by the whole world.
This is of course the kernel of the Jewish messianic hope, which we not only proclaim but must help along by our own efforts for civilisation.
Ramban says that the God who showed Israel His saving power at the Red Sea will always protect the righteous and punish the wicked.
To those who say, “But hasn’t God often let you down?” we reply, “There are better and worse moments in history, but in the end God’s word will prevail.”
That’s where the Shirah ends in a metaphorical sense, with the advent of the long awaited messianic redemption of Israel and mankind.