There were many manifestations. However, as the days passed, the memory of this unique day became dim. As Israel needed a time of remembering, so do we. He is among his people, then and now. Here God redeems a people for his own possession, shapes them, tells them how to live in covenant with him, and teaches them how to worship him.
Exodus is perhaps the most exciting story of redemption ever.
The short answer is that God loves his people and that He is in the redemption and recreating business. A good example is Jacob. Jacob consistently takes a lot of bad press from Christians. He is a schemer, a supplanter, a cheat. Further, God blessed him to an astonishing extent.