1) Why does God repeat the idea that the Locusts ate “the remainder” of the crops which Hail did not devastate?
2) Certainly, God was capable of destroying all of Egypt’s crops with the Plague of Hail. Why then did He leave over some crops for the locusts?
Question #1 suggests a relationship between Locusts and Hail. That is, Locusts addressed the same issue that Hail addressed. This must astonish man: a zoological plague follows a meteorological plague where both plagues targeted the destruction of Egypt’s crops. This sends a divine message of “orchestration”: there is one Governor of animal life and the heavens.
Egypt did not think that there was any connection between heavenly events, and those in the animal kingdom. They felt there exists two distinct spheres; each one controlled by distinct deities. God’s message here is that there is one Deity, and He controls all, expressed in a zoological phenomenon following a meteorological phenomenon that share the same objective. This connection is expressed as Locusts eating “the remainder” of the crops which Hail did not devastate. Both spheres of existence addressed the same goal, meaning a Designer orchestrated heaven and Earth to achieve a singular outcome. This taught Egypt that heaven and Earth are under the control of a single Deity.
God spared some crops from Hail’s destruction in order to teach Pharaoh and Egypt this very lesson. This is just one facet in God’s plan of the 10 Plagues to unveil Egypt’s idolatry as false.
 Exod. 10:5, 10:12, 10:15